SF: Fix freezing after follower display mode set

When a mode set is initiated for any display, SF skips committing until
HWC confirms the mode set by signaling the corresponding present fence.

However, if a concurrent follower display misses a frame (i.e. does not
signal the fence) as its mode is set, SF::commit skips endlessly, since
the Scheduler only checked for missed frames on followers after commit.

Fix this by having all displays FrameTargeter::beginFrame before commit.

Fixes: 301082260
Test: Mode set on external display does not sporadically freeze SF.
Test: SchedulerTest.onFrameSignalMultipleDisplays
Change-Id: Ic97f33975f6d30279cc65cefe5d7ccc2561c45aa