Camera: Documentation updates

 - Clarify hardware.camera feature being only for the back camera
 - Clarify what setting a CaptureRequest field to null does
 - Use preCorrectionActiveArray instead of activeArray in list of
   possible raw output sizes
 - Clarify length of GPS processing field for camera1 API

Bug: 24540625
Bug: 23908116
Bug: 23051627
Bug: 17345901
Change-Id: Iaf11fdf626268cf30f66b3628153ec3ac770c4f4