Add Quick Settings tile (#660)

* Add Quick Settings tile

- Add multi-client support to AmneziaVpnService
- Make AmneziaActivity permanently connected to AmneziaVpnService while it is running
- Refactor processing of connection state changes on qt side
- Add VpnState DataStore
- Add check if AmneziaVpnService is running

* Add tile reset when the server is removed from the application

client/android/src/org/amnezia/vpn/VpnState.kt

@@ -0,0 +1,75 @@
+package org.amnezia.vpn
+
+import android.app.Application
+import androidx.datastore.core.MultiProcessDataStoreFactory
+import androidx.datastore.core.Serializer
+import androidx.datastore.dataStoreFile
+import java.io.InputStream
+import java.io.ObjectInputStream
+import java.io.ObjectOutputStream
+import java.io.OutputStream
+import java.io.Serializable
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.withContext
+import org.amnezia.vpn.protocol.ProtocolState
+import org.amnezia.vpn.protocol.ProtocolState.DISCONNECTED
+import org.amnezia.vpn.util.Log
+
+private const val TAG = "VpnState"
+private const val STORE_FILE_NAME = "vpnState"
+
+data class VpnState(
+    val protocolState: ProtocolState,
+    val serverName: String? = null,
+    val serverIndex: Int = -1
+) : Serializable {
+    companion object {
+        private const val serialVersionUID: Long = -1760654961004181606
+        val defaultState: VpnState = VpnState(DISCONNECTED)
+    }
+}
+
+object VpnStateStore {
+    private lateinit var app: Application
+
+    private val dataStore = MultiProcessDataStoreFactory.create(
+        serializer = VpnStateSerializer(),
+        produceFile = { app.dataStoreFile(STORE_FILE_NAME) }
+    )
+
+    fun init(app: Application) {
+        Log.v(TAG, "Init VpnStateStore")
+        this.app = app
+    }
+
+    fun dataFlow(): Flow<VpnState> = dataStore.data
+
+    suspend fun store(f: (vpnState: VpnState) -> VpnState) {
+        try {
+            dataStore.updateData(f)
+        } catch (e : Exception) {
+            Log.e(TAG, "Failed to store VpnState: $e")
+        }
+    }
+}
+
+private class VpnStateSerializer : Serializer<VpnState> {
+    override val defaultValue: VpnState = VpnState.defaultState
+
+    override suspend fun readFrom(input: InputStream): VpnState {
+        return withContext(Dispatchers.IO) {
+            ObjectInputStream(input).use {
+                it.readObject() as VpnState
+            }
+        }
+    }
+
+    override suspend fun writeTo(t: VpnState, output: OutputStream) {
+        withContext(Dispatchers.IO) {
+            ObjectOutputStream(output).use {
+                it.writeObject(t)
+            }
+        }
+    }
+}