Background activity starts are now blocked by default.

Activity starts will now be blocked unless enabled in developer settings.

Bug: 129955487
Test: atest BackgroundActivityLaunchTest
Test: atest RootWindowContainerTests
Test: atest WmTests:ActivityStarterTests
Test: atest CtsWindowManagerDeviceTestCases:ActivityStarterTests
Test: atest CtsAppTestCases:.ServiceTest
Change-Id: I87855ee5b4e28a7d15eedd04e8b2bd5c0965bd66