Remove the ActivityClientRecord reference once destroyed

The ActivityClientRecord was referenced in ActivityThread
#mNewActivities even after it was destroyed. That was possible
when the app main thread is busy without idle reported.

Use ArrayList to easier maintain the activities to report idle.

Bug: 161230418
Test: verified on sample app
Change-Id: I8a8c49da9d942bb6efdfd29dba5495eed896209c