Clean up SyncManager.

1. Remove unused variables
2. Remove random selection of new job ID and make sure we always exit
   the loop. New job IDs will be selected more deterministically.
3. Query pending system jobs only once instead of for every potential
   new job ID.
4. Switch from iterator to basic for loop.

Bug: 169959994
Test: atest ContentResolverTest (all)
Test: atest CtsSyncManagerTest
Test: atest SyncOperationTest
Test: atest SyncManagerTest
Test: atest SyncRequestTest
Change-Id: Iff0f8edba8d424075f64a95e523b8c0c986aab33