SurfaceFlinger: Scheduler: move reset callback to scheduler thread

Dispatch timer reset callback from Scheduler's thread. This would
make both Scheduler callbacks (idle timer and reset timer) to be dispatched
from the same context and avoid any potential deadlocks.

Test: surfaceflinger unit tests
Change-Id: Iad7d10b85db090662f9b7c4ffb45ed9110035ea4