fs_mgr_fstab: Refactor & cleanup DSU mounting logic

* Instead of copy-and-modify `/data` mount entry, TransformFstabForDsu()
  can just modify all `/data` mount entries in-place.
* This also stops TransformFstabForDsu() from shuffling the mount entry
  of `/data` in fstab, simplifying the testcase.
* Implement GetEntriesForMountPoint() with GetEntriesByPred().
* Remove unused method BuildDsuUserdataFstabEntry() and
  EraseFstabEntry().

Bug: 235111004
Test: atest CtsFsMgrTestCases
Test: Presubmit GSI boot test
Change-Id: I4afb443b7e527038fb42424543bb1538b01d5ec9