Remove duplicated field of activity stopped state

It is leftover since merging AppWindowToken and ActivityRecord.
Both "stopped" and "mAppStopped" are set to true if the activity
completed stopped, and set to false if the activity is going to
resume. While "mAppStopped" will change to false if the activity
is requested to be visible. That is closer to the definition
about the relation of visibility and lifecycle state, so choose
to keep "mAppStopped".

Also inline notifyAppStopped() into activityStopped() because there
is only one usage since the methods were migrated from 2 files for
handling stopped activity.

Bug: 163976519
Test: CtsWindowManagerDeviceTestCases
Change-Id: Ieb2b4ebf91c08be1977bf619981454c8222ac20d