Expose Adapter.

We sometimes want just an adapter object instead of Fragment,
so that it can be available from not-Fragment UI
(like Spinner and ListView).

See I5e3d7ee721d1f0ec81dc9825b0d05f8faa731b80 for example usage.

Bug: 3133251
Change-Id: Ief6f25e9bc51c739b5cd2f2ba0afec5d2c16dc5b