Avoid layout passes when updating data activity indicators.

ImageView can optimize out layout passes if old and new drawable have
equal sizes. Take advantage of that by displaying a transparent drawable
with the same size as the in/out drawables when there's no activity
instead of clearing out the drawable in that case.

Needs https://gerrit.slimroms.net/3678

Change-Id: Iaaa556ffab3ed4be1da8016ed40d0a9c2efcde01