Update AppSearch code from Jetpack.

* 31db05: Move visibility settings into SetSchemaRequest.
* af6646: Support retrieving the current database's schema.
* 468ec2: Minor comment fix.
* b67963: Refactor AppSearchImpl to use List instead of Set.
* 1ae2bc: Delete an assertion that bundle.equals(bundle).

Bug: 169883602
Bug: 172260083
Bug: 162450968
Bug: 174825775
Test: Presubmit
Change-Id: I485bc9d1d9980467f6bdde1f36b077c0a3e7f5b4