New Executor implementaiton for SystemUI.

The implementation is DelayableExecutor. It looks and feels a lot like
a Handler (it is currently powered by a Handler under the covers).

Developers can now as for an Executor to be injected. By default, this
injects the "Background" Executor, but they can also ask for the
@Main or @Background Executor explicitly.

A FakeExecutor implementation is provided that implements a
synchronous version of DelayableExecutor. It can be configured to run
everything passed to it immediately, or can queue items to run in a
controlled, precise manner.

The new Executor is substituted into one place - TileQueryHelper -in
place of Handlers as a demonstration of its use. The test for
TileQueryHelper no longer requires the @RunWithLooper annotation as a
result.

Bug: 145135056
Test: atest SystemUITests
Change-Id: I76a8a9b2efa4c49e9b06492aca5b336726dc380e