Cleanup ActivityStack#requestFinishActivityLocked

In an attempt to reduce the number of different finish* methods in
ActivityTaskManager codebase this CL consolidates
ActivityStack#requestFinishActivityLocked() into
ActivityRecord#finishActivityLocked().

It was used before to initiate activity finishing and the return
value indicated if the request was successful. It was different from
the return value of ActivityRecord#finishActivityLocked(), which only
returned 'true' if the activity record was removed from history
immediately. In order to accomodate both, this CL updates the finish
method in ActivityRecord to return an integer status value, so it can
now be used in both cases.

Bug: 137329632
Test: atest WmTests:ActivityRecordTests
Change-Id: I30ebc306637dea5e8b28ca4b4dfaab8df31d2be3