Fix startFaceDetection RuntimeException.

Sometimes startFaceDetection is called twice and exception
is thrown.

Reproduction steps:
1. Set screen lock to none.
2. Start camera app and press power key to turn off screen immediately.
3. Start camera app.

bug:5497940
Change-Id: I3337d70e473943f5306cb459ce79c0faa45b77af
1 file changed