android: Add Quick Settings tile to toggle VPN state

Only if there is no currently active (or previously active) profile does
this currently operate on the configured (or stored most recently used)
profile.  This way it's possible to use a different connection and
quickly disable and re-enable it again.  When unlocked the profile name
is shown, when locked a generic text is used (this detection doesn't seem
to work 100% reliably).  To disconnect, the user is forced to unlock the
device, connecting is possible without, if the credentials are available
and no fatal error occurs (it even works with the system credential store,
at least on Android 8.1).

Note that the tile is not available right after a reboot.  It seems that
the system has to be unlocked once to activate third-party tiles (will
be interesting to see how this works together with Always-on VPN).

src/frontends/android/app/src/main/AndroidManifest.xml

@@ -36,6 +36,9 @@
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
+            <intent-filter>
+                <action android:name="android.service.quicksettings.action.QS_TILE_PREFERENCES" />
+            </intent-filter>
         </activity>
         <activity
             android:name=".ui.VpnProfileControlActivity"
@@ -156,6 +159,15 @@
                 <action android:name="android.net.VpnService" />
             </intent-filter>
         </service>
+        <service
+            android:name=".ui.VpnTileService"
+            android:label="@string/tile_default"
+            android:icon="@drawable/ic_notification"
+            android:permission="android.permission.BIND_QUICK_SETTINGS_TILE">
+            <intent-filter>
+                <action android:name="android.service.quicksettings.action.QS_TILE" />
+            </intent-filter>
+        </service>
 
         <provider
             android:name=".data.LogContentProvider"