Defer ending launcher power mode hint until after transient transition

Because transient launch will resume the target activity, the power
mode will be cleared after the activity reported idle. But the
animation may be still running that causes some frame drops by lower
cpu frequency. So defer it until the transition is finished.

The power mode will be cleared from:
Transition#finishTransition
 > scheduleProcessStoppingAndFinishingActivitiesIfNeeded
  > allResumedActivitiesIdle
   > endLaunchPowerMode

Bug: 291284150
Test: atest TransitionTests#testTransientLaunch
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:cf2dfc3e29be449078439fdc6a7317cf1944ce8a)
Merged-In: If664fbc58554d7cfabae86c9ec282f3f7cf50526
Change-Id: If664fbc58554d7cfabae86c9ec282f3f7cf50526