Don't block activity starts in permissive mode.

When activity starts have been enabled via developer options we
shouldn't block them. But we still want to show a toast to warn that
the start would have been blocked.

Scenarios where we were blocking when we shouldn't:
- Creating a new task from a background receiver (here the activity is
  created but not brought to the front).
- Starting an activity while app switching protection is in force.
- Various cases when starting an activity in a new task or a task that
  doesn't already have one of our activities in it.

Bug: 132919170
Bug: 132919170
Test: manual
Test: atest BackgroundActivityLaunchTest
Test: atest RootWindowContainerTests
Test: atest WmTests:ActivityStarterTests
Test: atest CtsWindowManagerDeviceTestCases:ActivityStarterTests
Test: atest CtsAppTestCases:.ServiceTest
Change-Id: I32d75d04ea93cb4883e93bbe74468755a708ca24