Add AbstractThreadedSyncAdapter#onUnsyncableAccount API

This allows for an SyncAdapter to defer all onPerformSync calls (and the
admission check for those) until the adapter is ready to accept those
calls.

Test: atest android.content.cts.AccountAccessSameCertTest
      atest android.content.cts.ContentResolverSyncTestCase
      atest android.content.cts.DeferSyncTest
      atest com.android.cts.content.CtsSyncAccountAccessOtherCertTestCases
      Set up a user and work account and waited until their gmail
      synced.

Bug: 72459220
Change-Id: I8f9ab735d64189578ccdd1c3e1d7b7c5383b8ee9