SF: avoid rearming Timer during cancel

Averts a rare race condition by which a callback would be moved back a
vsync period inappropriately by rearming the callback-dispatching timer
only when the next-up callback was the one that was cancelled.

This was an analagous problem (but through the cancel() path) to
I0e7e2e3698ed6d1765082db20d9cf25f6e6c2db2

Test: 2 new unit tests
Test: boot to home, inspect for problems
Test: dogfood patch on device
Test: A/B uibench sanity check
Fixes: 154303580

Change-Id: I02b2ba12623ac683d9b1c592fdc35e7c7494261a