Update Pip launches to not enter pinned task if in background.

Addresses a BAL bypass where Pip could be started without the launcher
being visible.

Bug: 271576718
Test: atest CtsWindowManagerDeviceTestCases:PinnedStackTests
Test: atest android.server.wm.BackgroundActivityLaunchTest#testPipCannotStartFromBackground
Change-Id: Icfe0a17d7f6f127acaae8400a97e8bdc53fcc9ad