Move GnssStats puller to binder thread

Pass direct executor to setPullAtomCallback. This causes the puller to
run directly on the binder thread, as opposed to the Background thread,
which could be busy with other work.

This change means that the puller can be executed concurrently. To
properly handle that, the Statistics class is made thread-safe.

Test: adb shell cmd stats pull-source 10074
Bug: 157186182
Change-Id: I6c0940fd4ed231f9d962981cc1f9e8c983c48f7f