Add shortcut for tether offload

Fix #41.

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

@@ -0,0 +1,27 @@
+package be.mygod.vpnhotspot.net
+
+import android.provider.Settings
+import androidx.annotation.RequiresApi
+import be.mygod.vpnhotspot.App.Companion.app
+import be.mygod.vpnhotspot.util.RootSession
+
+/**
+ * It's hard to change tethering rules with Tethering hardware acceleration enabled for now.
+ *
+ * See also:
+ *   android.provider.Settings.Global.TETHER_OFFLOAD_DISABLED
+ *   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 {
+    private const val TETHER_OFFLOAD_DISABLED = "tether_offload_disabled"
+    @JvmStatic
+    var enabled: Boolean
+        get() = Settings.Global.getInt(app.contentResolver, TETHER_OFFLOAD_DISABLED, 0) == 0
+        set(value) {
+            RootSession.use {
+                it.exec("settings put global $TETHER_OFFLOAD_DISABLED ${if (value) 0 else 1}")
+            }
+        }
+}