Sync AppSearch from framework.

Included changes:
* 9d1cf52: Supports schema migration in AppSearch
* 1c55f12: Change AppSearch setSchema() to return a SetSchemaRsponse.
* 9d94171: Unhide AppSearchMigrationHelper, Transformer and
           SetSchemaResponse.
* f07132f: add maybeFlush to AppSearchSession.

Bug: 169883602
Bug: 173742929
Test: Presubmit

Change-Id: I9e70ed04014ae3d134defb93c5cb02a5def0c71e