Bluetooth: log unexpected crashes and restarts

Add some reasons to the log when:
 - the Bluetooth stack crashes underneath us
 - user switches
 - we automatically restart

List every instance of the stack crashing underneath us in the dumpsys.
Clean up some unused constants.

Test: notice a Bluetooth crash, adb bugreport
Change-Id: I79195f0a574de1bf48a976d31c64084d3ce2e14a