Camera2: fix unresponsive shutter button after failure

Shutter button is disabled during capture(). It's only re-enabled in
onPicutreTaken() callback. If camera is switched just before counting
down finishes, picture taking may fail and shutter button is left
disabled. Enabling shutter button in onPreviewStarted() fixes the
problem.

Change-Id: I92c644e9a0d283f9ab0269b85586f27e0f4eeb1b
CRs-Fixed: 667311