GuidedStepFragment Refactoring background handling

Changed the way background works, no longer use a dedicated
BackgroundFragment, make background part of GuidedStepFragment.
This need some tuning to make background view part of
GuidedStepFragment transition (and excluded in some cases).

Switch the default Ui Style to be UI_STYLE_ENTRANCE, allows
app launches GuidedStepFragment without using helper method.

Bug: 25671370

Change-Id: Ic68c0cc796d32df6919593d391a6fcfae99258ae
7 files changed