Add AwVariationsSeedFetchService and refactory VariationsSeedFetcher

android_webview/apk/java/AndroidManifest.xml

Index: android_webview/apk/java/AndroidManifest.xml
diff --git a/android_webview/apk/java/AndroidManifest.xml b/android_webview/apk/java/AndroidManifest.xml
index fd3ca2b367066aeeb8a559e191d365d8402d84e3..ae3696851ff138ffdfeec3a4ddec2c9a5d5e10cc 100644
--- a/android_webview/apk/java/AndroidManifest.xml
+++ b/android_webview/apk/java/AndroidManifest.xml
@@ -39,6 +39,10 @@
                       android:exported="true"
                       android:authorities="{{ manifest_package }}.LicenseContentProvider" />
             {% if donor_package is not defined %}
+                <service android:name="org.chromium.android_webview.AwVariationsSeedFetchService"
+                         android:permission="android.permission.BIND_JOB_SERVICE"
+                         android:exported="true"
+                         android:process=":variations_service"/>
                 <service android:name="org.chromium.android_webview.crash.CrashReceiverService"
                          android:exported="true"
                          android:process=":crash_receiver_service"/>