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