Only persist last Shared Preferences state

If multiple async shared preferences writes are queued, all but the
last one can be ignored as they will be overwritten by the last one
anyway.

For commit() we need to make sure that we have at least persisted the
state of the commit.

Generation counts are 64 bit, hence they never overflow.

Test: Produced a lot of SharedPreferences.Editor.apply and did not see
excessive writes anymore, ran SharedPreferences CTS tests
Bug: 33385963
Change-Id: I3968ed4b71befee6eeb90bea1666a0bb646544f6