Camera2: Check mCameradevice before setting skinToneEnhancement

Null pointer exception in app is observed when we open camera
and immediately press home/back button. This is because
mCameradevice will be null by the time handler thread gets a
message to setskinToneEnhancement parameter.

( cherrypicked from commit 06fee777df529fa53ac5c5305e7324ad1baf085d )

Change-Id: I7f216a11a29b6b205be4c2d0ca8f1c346284c294
CRs-Fixed: 547522