TransformFstabForDsu: Support erofs GSI and refactor

Synthesis mount entry for erofs and ext4.
Fix the synthesis logic to always create a new entry from scratch. The
old logic of "copy an existing mount entry and edit its fs_type"
doesn't work because different fs_type would have incomptable
fs_options. For example, changing this:
  system  /system  ext4  ro,barrier=1  wait,logical,first_stage_mount
to this:
  system  /system  erofs  ro,barrier=1  wait,logical,first_stage_mount
doesn't work, because the erofs driver won't recognize the "barrier=1"
mount flag, since "barrier=1" is specific to ext4.

Bug: 235111004
Test: Boot erofs GSI.
Test: atest CtsFsMgrTestCases
Change-Id: I57132a55a089c7aae3e17c717ecd9dc1047fede8