Prevent crash on alarms going off with a different user active

[davidgyoung]

This attempts to fix #876, which reports a SecurityException crash when a library Alarm goes off after somebody switched users on the phone. The fix is to detect the user switch and cancel the alarm.

Testing this on the Google Pixel 3a, I start the reference app (with a foreground service enabled), then switch to the Guest User. I see that the logs show the alarm gets cancelled:

05-29 13:44:14.588  3760  3760 W CycledLeScanner: User switch detected.  Cancelling alarm to prevent potential crash.
05-29 13:44:14.588  3760  3760 D CycledLeScanner: cancel wakeup alarm: PendingIntent{b0fb482: android.os.BinderProxy@d3ba793}
05-29 13:44:14.600  3760  3760 D CycledLeScanner: Set a wakeup alarm to go off in 9223372035236175056 ms: PendingIntent{b0fb482: android.os.BinderProxy@d3ba793}

As a side note, this test showed some fascinating things about how the library behaves after user switching, at lest on a Pixel 3a with Android 9.0 and the library running with a foreground service:

1. The library keeps running after the user switch, and I see logs coming from the foreground service started by an app from the primary user.
2. Bluetooth is switched off automatically after the user switch happens (I guess this is the Pixel 3a default behavior?) so beacon detections stop.
3. When switching back to the primary user, bluetooth comes back on like before, and detections resume as if nothing happened.

Maybe the above is expected behavior, but I was shocked to see that all work! I figured I'd note it for the record.