Porting the async AppSearchSession to platform.

AppSearchSession has different context in platform, it's not directly
ported from jetpack.

This change switch AppSearch in platform from synchronize to async.
AppSearch platform Async design doc:
https://docs.google.com/document/d/1YGrM4s65ZhWy0P1zdI1ssfjWuCctOKxdxUKzUYS5MyY/edit?usp=sharing

Changes included:
#89b527b: Replace AppSearchManager with a SearchSession.

Bug: 162450968
Test: presubmit
Change-Id: I4c7e1b618b6f8eab2087813c32d15339b3daa8ab