Run migration in background service.

In order to avoid a half done migration we are moving the migration to a background service (that is
running in the "foreground").

This is the Fenix part of:
https://github.com/mozilla-mobile/android-components/issues/4879

app/src/migration/AndroidManifest.xml

@@ -12,6 +12,7 @@
     <application
         android:name="org.mozilla.fenix.MigratingFenixApplication"
         tools:replace="android:name">
+        <service android:name="org.mozilla.fenix.MigrationService" />
     </application>
 </manifest>
 

app/src/migration/java/org/mozilla/fenix/MigratingFenixApplication.kt

@@ -4,6 +4,7 @@
 
 package org.mozilla.fenix
 
+import android.content.Context
 import kotlinx.coroutines.runBlocking
 import mozilla.components.support.migration.FennecMigrator
 
@@ -11,12 +12,25 @@ import mozilla.components.support.migration.FennecMigrator
  * An application class which knows how to migrate Fennec data.
  */
 class MigratingFenixApplication : FenixApplication() {
+    val migrator by lazy {
+        FennecMigrator.Builder(this, this.components.analytics.crashReporter)
+            .migrateOpenTabs(this.components.core.sessionManager)
+            .migrateHistory(this.components.core.historyStorage)
+            .migrateBookmarks(this.components.core.bookmarksStorage)
+            .migrateLogins(
+                this.components.core.passwordsStorage.store,
+                this.components.core.passwordsEncryptionKey
+            )
+            .migrateFxa(this.components.backgroundServices.accountManager)
+            .build()
+    }
+
     override fun setupInMainProcessOnly() {
         migrateGeckoBlocking()
 
         super.setupInMainProcessOnly()
 
-        migrateDataAsynchronously()
+        migrator.startMigrationServiceIfNeeded(MigrationService::class.java)
     }
 
     private fun migrateGeckoBlocking() {
@@ -28,19 +42,8 @@ class MigratingFenixApplication : FenixApplication() {
             migrator.migrateAsync().await()
         }
     }
-
-    private fun migrateDataAsynchronously() {
-        val migrator = FennecMigrator.Builder(this, this.components.analytics.crashReporter)
-            .migrateOpenTabs(this.components.core.sessionManager)
-            .migrateHistory(this.components.core.historyStorage)
-            .migrateBookmarks(this.components.core.bookmarksStorage)
-            .migrateLogins(
-                this.components.core.passwordsStorage.store,
-                this.components.core.passwordsEncryptionKey
-            )
-            .migrateFxa(this.components.backgroundServices.accountManager)
-            .build()
-
-        migrator.migrateAsync()
-    }
+}
+
+fun Context.getMigratorFromApplication(): FennecMigrator {
+    return (applicationContext as MigratingFenixApplication).migrator
 }

app/src/migration/java/org/mozilla/fenix/MigrationService.kt

@@ -0,0 +1,14 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix
+
+import mozilla.components.support.migration.AbstractMigrationService
+
+/**
+ * Background service for running the migration from legacy Firefox for Android (Fennec).
+ */
+class MigrationService : AbstractMigrationService() {
+    override val migrator by lazy { getMigratorFromApplication() }
+}