fs_mgr: add fs_mgr_read_fstab_with_dt() API

With the early mount support in init, fstab entries of verified partitions
(e.g., /system, /vendor) will be moved into device tree in kernel image.

Those early mount fstab entries will be removed from the fstab file to
prevent duplicated and/or inconsistent settings.

This change adds a new function: fs_mgr_read_fstab_with_dt(), to return
the combined results from both places. It also removes
fs_mgr_read_fstab_file() from the public APIs and makes it as an
internal function.

Bug: 35811655
Test: early mount /vendor with dm-verity on sailfish

Change-Id: I2fba3614685bf9f852a789cda6f53013e2164e60