Fix empty stacks leaked after blocking background activity launches.

Background activity starts were blocked while stacks were created
and leaked.

Remove the stack if the activity failed to launch and no activities
left in the stack. Also avoid creating new stack for the blocked
background activity which is unnecessary.

Bug: 134016361
Test: atest BackgroundActivityLaunchTest

Change-Id: Ifa04a9062cb771929a9614cc164fd64b3f60edf1