Adding GSI fstab entries when needed

In ReadFstabFromFile(), currently it always adds 'system_gsi' and
'userdata_gsi' fstab entries when running in Live GSI. As the API
might be used to load a fstab file without "/system" and/or "/data",
it's better to replace "/system" with 'system_gsi' and "/data" with
'userdata_gsi', instead of adding 'system_gsi' and 'userdata_gsi'
unconditionally.

Bug: 124640105
Test: boot a Live GSI, then `atest libfs_avb_device_test`

Change-Id: I52928f95b9ebd12ce09ffd538caf96a2de430dbc