Graphics depth formats: Add confidence values

Supplement the dense and sparse depth image formats with
confidence values.

For {HAL_PIXEL_FORMAT_Y_16, HAL_DATASPACE_DEPTH}, use the 3 MSBs
for confidence.

For {HAL_PIXEL_FORMAT_BLOB, HAL_DATASPACE_DEPTH}, add a 4th float
value for confidence.

Bug: 20123879
Change-Id: Idc27081efec682a712ce77ccfec59328f298b944