fix shell available detection in DeviceStateMonitor

original implementation checks for substring 'system', but this
fails to detect recovery mode, since you'll get:
- exec '/system/bin/sh' failed: No such file or directory (2) -

changed to check for /system/bin/adb instead

Change-Id: Ied5dd9ce2988b67fb76ae9bf2cc8301828c8a90e
1 file changed