Delay SharedPreferences.apply() by 50 ms

... so that multiple applies can be combined into a single write.

Do do this I replaces the executor in the QueuedWork by a more Android-y
handler.

Test: Ran shared preferences CTS tests. Looked at log and saw a lot of
      skipped writes
Bug: 33385963

Change-Id: I8f33df717be7091532930ccf6ca8c48940e4edd4