Remove meaningless ActivityStarter#mReuseTask

It was also assigned when reusing a cleared task. But the latest
only assign it from mInTask, which has lost it original purpose.

Add a explicit mIsTaskCleared for notifyActivityRestartAttempt.
The condition 'clearedTask' is removed because it was almost always
not satisfied.

Bug: 163976519
Test: WindowManagerServiceTests
Change-Id: I6e3cd10463cacc4aa7fb84a1f631c0179ea4e013