Don't consider tasks with INITIALIZING top activity as visible

To fix b/130645908, tasks with top activities in INITIALIZING state were
considered visible. This allowed apps to circumvent background activity
launch restriction by starting 2 activities when in a background stack.
The first activity will remain in INITIALIZING state (since it's in the
bg), so the second launch is permitted since top activity is
INITIALIZING.

Removing '|| topActivity.isState(INITIALIZING)' removes the
vulnerability and curiously still allow the legitimate use case of the
bug linked above to work, i.e. the test in ag/7088262 still passes. This
is because of 2 reasons:
1) Grace period introduced in ag/7638265.
2) Allow bg activity starts to put activity on top of bg task (instead of
   fg’ing it) in ag/7190176.

I verified (2) by removing the grace period check, reverting the linked
CL and verifying that the test failed.

This essentially reverts ag/7090415.

Test: atest BackgroundActivityLaunchTest \
          RootWindowContainerTests \
          WmTests:ActivityStarterTests \
          CtsWindowManagerDeviceTestCases:ActivityStarterTests
Bug: 138583650
Bug: 130645908
Change-Id: Iee4ba841c6c83888b55c5cb55fffc949048d2030