GuidedStepFragment: fix temporary black screen in activity enter

Last CL deprecating background view causes the regression.

We need to keep the activity translucent during fade-in background view
of GuidedStepFragment. The trick is to actually set a transparent ColorDrawable
and make it last as long as the background view fade-in.

Change-Id: I53e63fe0cebf60028dc8f45dc3446407ad7a5ef0
3 files changed