adb: Look for executables in alternate places

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

 * Use /sbin for path to 'sh' and 'bu' when in recovery mode.

 * Honor persist.sys.adb.shell when not in recovery mode.

Change-Id: I02ac651397021097fe865436c982e2b720d7917f