Write on-device integration tests for Android notifications

[gnprice]

This is part of #757. As discussed in that issue, Android notifications are one of the few areas where we ought to have integration tests running on real devices, and are probably the top priority among those areas.

A prime example of this was #755, where it turned out that Android 11 and older (but not Android 12+) had a quirk that caused empty summary notifications to stick around after the notifications they contain were removed, unless the seemingly-irrelevant autoCancel flag was set (by calling setAutoCancel(true)). @rajveermalviya found that through patient manual testing; it would have been great to be able to find it through an automated test.