Make ImageView drawable update more efficient in some cases.

When updating the view's drawable via setImageResource or setImageURI,
make sure to skip the layout pass if the old and new drawable have equal
sizes. This follows what is already done for setDrawable().

Change-Id: If73d225e38d0a2f7c616c4aeb33f67bdaed390f1
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.

Change-Id: Iaaa556ffab3ed4be1da8016ed40d0a9c2efcde01