Move engaged session state cleanup to destroy method

This is where it effectively happened before ag/33469175
and it needs to stay there to ensure there are no other
actions in MediaSessionService that can undo the cleanup.

This fixes a test failure introduced by ag/33469175 that
triggers this issue.

Test: atest ActivityManagerNotifyMediaFGSTypeTest#testAppInBgWithActivePlayingMediaSessionAndNotificationReleaseItsSessionGoesToBg
Bug: 417529555
Flag: EXEMPT bugfix
Change-Id: I233c0c6e764ddc027aba6225bd4fabdc81019e6f