Merge AppSearchObserver into framework.

Changes included:
* 166f03: Add an API to create and register an Observer for AppSearch changes.
* ff3bc6: Hook up AppSearchObserverCallback to be triggered on document puts.
* 36f729: Hook up removeById to send change notifications to AppSearchObserver.
* 2c0e6e: Hook up removeByQuery to send change notifications to AppSearchObserver.
* b00804: Rework threading and concurrency in AppSearchObserver.

Bug: 193494000
Test: GlobalSearchSessionCtsTest
Change-Id: Icb1cfe0d06c0f019349afa755ff7773e7691c515
18 files changed