Attempt to detect tether offload support dynamically on Android 8-

Addresses #224.

README.md

@@ -277,7 +277,7 @@ Greylisted/blacklisted APIs or internal constants: (some constants are hardcoded
 * [`Landroid/net/wifi/p2p/WifiP2pManager;->deletePersistentGroup(Landroid/net/wifi/p2p/WifiP2pManager$Channel;ILandroid/net/wifi/p2p/WifiP2pManager$ActionListener;)V,system-api,whitelist`](https://android.googlesource.com/platform/prebuilts/runtime/+/4601d91/appcompat/hiddenapi-flags.csv#158296)
 * [`Landroid/net/wifi/p2p/WifiP2pManager;->requestPersistentGroupInfo(Landroid/net/wifi/p2p/WifiP2pManager$Channel;Landroid/net/wifi/p2p/WifiP2pManager$PersistentGroupInfoListener;)V,system-api,whitelist`](https://android.googlesource.com/platform/prebuilts/runtime/+/4601d91/appcompat/hiddenapi-flags.csv#158320)
 * [`Landroid/net/wifi/p2p/WifiP2pManager;->setWifiP2pChannels(Landroid/net/wifi/p2p/WifiP2pManager$Channel;IILandroid/net/wifi/p2p/WifiP2pManager$ActionListener;)V,system-api,whitelist`](https://android.googlesource.com/platform/prebuilts/runtime/+/4601d91/appcompat/hiddenapi-flags.csv#158330)
-* (since API 27) [`Landroid/provider/Settings$Global;->TETHER_OFFLOAD_DISABLED:Ljava/lang/String;,system-api,test-api,whitelist`](https://android.googlesource.com/platform/prebuilts/runtime/+/4601d91/appcompat/hiddenapi-flags.csv#183757)
+* [`Landroid/provider/Settings$Global;->TETHER_OFFLOAD_DISABLED:Ljava/lang/String;,system-api,test-api,whitelist`](https://android.googlesource.com/platform/prebuilts/runtime/+/4601d91/appcompat/hiddenapi-flags.csv#183757)
 
 </details>
 

mobile/src/main/java/be/mygod/vpnhotspot/SettingsPreferenceFragment.kt

@@ -48,7 +48,7 @@ class SettingsPreferenceFragment : PreferenceFragmentCompat() {
         SummaryFallbackProvider(findPreference(UpstreamMonitor.KEY)!!)
         SummaryFallbackProvider(findPreference(FallbackUpstreamMonitor.KEY)!!)
         findPreference<SwitchPreference>("system.enableTetherOffload")!!.apply {
-            if (Build.VERSION.SDK_INT >= 27) {
+            if (TetherOffloadManager.supported) {
                 isChecked = TetherOffloadManager.enabled
                 setOnPreferenceChangeListener { _, newValue ->
                     if (TetherOffloadManager.enabled != newValue) viewLifecycleOwner.lifecycleScope.launchWhenCreated {

mobile/src/main/java/be/mygod/vpnhotspot/manage/ManageBar.kt

@@ -2,7 +2,6 @@ package be.mygod.vpnhotspot.manage
 
 import android.content.Context
 import android.content.Intent
-import android.os.Build
 import android.view.View
 import androidx.databinding.BaseObservable
 import androidx.recyclerview.widget.RecyclerView
@@ -17,7 +16,7 @@ object ManageBar : Manager() {
     private const val SETTINGS_2 = "com.android.settings.TetherSettings"
 
     object Data : BaseObservable() {
-        val offloadEnabled get() = Build.VERSION.SDK_INT >= 27 && TetherOffloadManager.enabled
+        val offloadEnabled get() = TetherOffloadManager.supported && TetherOffloadManager.enabled
     }
     class ViewHolder(binding: ListitemManageBinding) : RecyclerView.ViewHolder(binding.root), View.OnClickListener {
         init {

mobile/src/main/java/be/mygod/vpnhotspot/net/TetherOffloadManager.kt

@@ -1,9 +1,10 @@
 package be.mygod.vpnhotspot.net
 
+import android.os.Build
 import android.provider.Settings
-import androidx.annotation.RequiresApi
 import be.mygod.vpnhotspot.App.Companion.app
 import be.mygod.vpnhotspot.root.SettingsGlobalPut
+import timber.log.Timber
 
 /**
  * It's hard to change tethering rules with Tethering hardware acceleration enabled for now.
@@ -13,8 +14,11 @@ import be.mygod.vpnhotspot.root.SettingsGlobalPut
  *   https://android.googlesource.com/platform/frameworks/base/+/android-8.1.0_r1/services/core/java/com/android/server/connectivity/tethering/OffloadHardwareInterface.java#45
  *   https://android.googlesource.com/platform/hardware/qcom/data/ipacfg-mgr/+/master/msm8998/ipacm/src/IPACM_OffloadManager.cpp
  */
-@RequiresApi(27)
 object TetherOffloadManager {
+    val supported by lazy {
+        Build.VERSION.SDK_INT >= 27 || Settings.Global::class.java.getDeclaredField("TETHER_OFFLOAD_DISABLED")
+            .get(null).also { if (it != TETHER_OFFLOAD_DISABLED) Timber.w(Exception("Unknown field $it")) } != null
+    }
     private const val TETHER_OFFLOAD_DISABLED = "tether_offload_disabled"
     val enabled get() = Settings.Global.getInt(app.contentResolver, TETHER_OFFLOAD_DISABLED, 0) == 0
     suspend fun setEnabled(value: Boolean) = SettingsGlobalPut.int(TETHER_OFFLOAD_DISABLED, if (value) 0 else 1)