ImageFormat: Update definition of depth formats for confidence.

DEPTH16: Use high 3 bits for confidence measurement, with
0 = 100% confidence, 1 = 0% confidence, 2 = 1/7 (14.3%) confidence, etc.

DEPTH_POINT_CLOUD: Add confidence as 4th entry to each point,
with a value in range 0.f - 1.f inclusive.

Bug: 20123879
Change-Id: I23317b922ac727751156fa418ed239a696a898e5