Fix non tailcall onTransitionFinished

According to current design, RecentsTransitionHandler's finish methods
are not re-entrant, so onTransitionFinished *must* be tail called from
RecentsTransitionHandler to avoid broken state when we come back.

Test: atest 'ShellTransitionTests#testTransitSleep_squashesRecents' # see follow-up commit
Bug: 277212616
Change-Id: I97fd08f16e096588e2c838b181ff0287410f8326