SurfaceFlinger: add explicit eEarlyWakeup start and end

The current handling of eEarlyWakeup is using 2 frames of timeout
in SurfaceFlinger to know when we are no longer in early offset config.
There are few cases where a transaction is dropped or delayed (usually
caused by the offset change itself) which results in switching back
and forth from early offset to non-early offset. This change adds two
new flags, eExplicitEarlyWakeupStart and eExplicitEarlyWakeupEnd that
will be used by WindowManager to indicate when to enter early offset
and when to exit. With these explicit flags, the timings on transaction
no longer matters and we consistently stay at early offset config for the desired
duration.

Bug: 158127834
Test: Quick switch between apps and verify that offset doesn't change
Test: adb shell /data/nativetest64/libsurfaceflinger_unittest/libsurfaceflinger_unittest
Change-Id: Ie10af30a2d8c0f4f21ac7ffed469a74e1bf8dbc1