Adding Launcher3 callbacks for trimming memory.

It will help reduce memory footprint of Launcher, when in background.
It is required to help deallocate images, when Google Now Launcher
goes into the background. See cl/83222937/ .

Plan after this CL:
- Generate drop CL with launcher3_drop.sh. Add empty metods to
 make code compile.
- Sync the cl/83222937 and hide the code behind the feature flag.

It was previously submitted in ag/628070 and reverted in ag/630423
due to GSA release schedule.

Change-Id: I56eee618b86d518d0afc362b6cf44c3bbec1f19c