Describe image loads in Traces

This adds tracing to ImageDecoder for image loads. When a drawable is
being loaded, it'll emit a short description of the resource and desired
loading sizes. This will make it significantly easier to find large,
slow bitmap loads when debugging regressions.

Bug: 235451049
Test: physically on a Raven device
Change-Id: I615342f1bd11d867e0bd33209507330848df8525