Initialize WorkManager ourself.

This gives us more control over when it happens, as well as lets us set
things like the debug level. Also let's us get rid of the synchronized
block we had in Application#onCreate().

AndroidManifest.xml

@@ -572,6 +572,12 @@
               android:authorities="org.thoughtcrime.securesms.database.attachment"
               android:exported="false" />
 
+    <provider
+              android:name="androidx.work.impl.WorkManagerInitializer"
+              android:authorities="${applicationId}.workmanager-init"
+              android:exported="false"
+              tools:node="remove" />
+
     <receiver android:name=".service.BootReceiver">
         <intent-filter>
             <action android:name="android.intent.action.BOOT_COMPLETED"/>