Allow second consecutive activity start.

If we start two activities in onCreate the second one was being
blocked - because the current activity is no longer top but the new
activity is not yet visible. So we treat an INITIALIZING activity
as being visible for this purpose.

Bug: 130645908
Test: atest BackgroundActivityLaunchTest RootWindowContainerTests \
WmTests:ActivityStarterTests \
CtsWindowManagerDeviceTestCases:ActivityStarterTests
Change-Id: I96749f8b4d385527c91725827880bd12371d2411