Add a test for START_ACTIVITIES_FROM_BACKGROUND permission.

The shell has this permissions, so we're using it to test it (it's a
system permission, we can't give it to a test app). The idea is to
disable all other ways that the shell is allowed to launch from the
background until this permission is the only thing that allows it to
work. I've tested whether I've accomplished this by locally removing the
permission from shell and observing this test fail.

Test: atest BackgroundActivityLaunchTest
Bug: 171196794
Change-Id: I8fec8a3ca5abb80831820b49b5208f06c82acb41