adb: Look for bu in /sbin when in recovery mode

 * Restore global variable recovery_mode.  This is set based on the
   commandline parameter --banner.

 * Use recovery_mode to select path to 'bu' binary for both backup and
   restore services.

Change-Id: Ie4c945d00601514d7f16357bae10ff7f232633e1
Merge remote-tracking branch 'cm/cm-12.0' into cm-12.0