BrowseFragment: fix bug when MainFragment is initialized early

If MainFragment was restored before BrowseFragment's adapter,
the HeadersFragment is empty and user may opens empty HeadersFragment
when pressing LEFT or BACK key.

This change prevent LEFT or BACK at the first place.  We cannot
prevent BACK from pop backstack:  so in the BACK case,
re-add the stack entry.

Change-Id: I112e746ddd9ac5c2dcc34dec461dce4c65f26f2d
2 files changed