adb: replacing fs_mgr_read_fstab() with fs_mgr_read_fstab_default()

The original default /fstab.{ro.hardware} might be moved to
/vendor/etc/. or /odm/etc/. Use the new API to get the default
fstab instead of using the hard-coded /fstab.{ro.hardware}.

Bug: 35811655
Test: boot marlin with /vendor/etc/fstab.marlin, then run 'adb remount'
Change-Id: I927209ce3c5bea45c01ed631a7c4c320fe728c00