Move GetMountedEntryForUserdata to fs_mgr.h

Logic of unwinding dm-device stack to figure out what entry was used to
mount userdata turned out to be a little bit more involved, and it
shouldn't be part of libfstab

This CL just moves code around and cleans API a little bit, actual fix
will be in the follow-up CL.

Test: atest CtsFsMgrTest
Test: atest CtsUserspaceRebootHostSideTestCases
Bug: 148612148
Change-Id: If0f8a765dba63adb0e6a711eb81fffdcabea3480