[DO NOT MERGE] 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
Bug: 30662828

Change-Id: I8f33df717be7091532930ccf6ca8c48940e4edd4
(cherry picked from commit 33496448)