AImageReader: Add support for private format

The private format was unsupported in API24, but since API26 added
getAHardwareBuffer for AImageReader, the format becomes useful now. It
maps to HAL_PIXEL_FORMAT_IMPLEMENTATION_DEFINED defined in
<system/graphics.h>.

This CL also updates the supported format/usage combination with the
rule that if format is AIMAGE_FORMAT_PRIVATE, neither CPU_READ or
CPU_READ_OFTEN is supported.

Bug: 37097572
Test: AImageReaderCts, Graphics CTS
Change-Id: Ide3db8c6ceb4504d6f198ea1d9526edecc3a56ad