GuidedStepFragment: API tuning

Hide all entry name related APIs.
Those are used by the helper functions add/addAsRoot/finishGuidedStepFragments/
popBackStackToGuidedStepFragment.
So apps either use the helper functions without knowing details of entry names,
or they are on own.

Change-Id: I2c0e1847bd25cc8db82488c87125ed20202aa6e8
3 files changed