Split auto dismissing functionality from heads up

Refactor HeadsUpManager so that the auto dismissing alert functionality
is separate and reusable for future use with ambient pulses.  Add tests
for this new class and also a few for HeadsUpMananger specific logic

First in two-part CL to split ambient pulse logic from heads up logic.
Second CL will add an ambient pulse manager which uses the alert logic
introduced here and involve more of the actual split.

Test: runtest systemui, manual
Change-Id: I08069562e7bea4c8e25674aa35237e1bb8cf4475