Add GapWorker task prioritization

Bug: 27106058
Test: new tests, existing caching tests still pass

GapWorker now breaks down all scheduled works (each prefetch itemview
from each RecyclerView) into tasks, which it sorts before starting.

In addition, ignore deadlines for items we expect to need next frame.

Change-Id: I23b5a37925bbf1e392d6c48f5341e1382f5fb89b
5 files changed