Move StatsPullAtomService pullers to binder thread

Pass in a direct executor when registering pullers from
StatsPullAtomService. This will execute the pullers on the binder
thread. This should decrease pull times when the background thread is
occupied.

Bug: 157186182
Test: atest CtsStatsdHostTestCases
Change-Id: I43273f882a5e5a08dca0e07ec5cfe533322a9671