Camera: Disable Camera Launcher when both Front and Back sensors are not detected

Instead of checking only back sensor, this change checks both Front and Back sensors
at the time of first time boot up and then if no sensors are present, disable all
camera activities/CameraLauncher.

Ported from http://review.evervolv.com/#/c/8926/

Change-Id: Ic9ebd3140caa8df079fe01553d62e96207f2e159
Camera: fix crash on screen off-on on preview screen

Some Camera HALs don't like stopPreview being called with
setPreviewTexture set to null.

Problem occurs on:
 - Acer A700 (Tegra 3) - always
 - Samsung I9082 (Broadcom) - sometimes

Change-Id: Ifd5375d69ef9851a058d2385bf2ee9c89e85bae0