init: Unify duplicated get_android_dt_dir with libfs_mgr

init and libfs_mgr both defines get_android_dt_dir() with subtle
differences. Merge the two implementations into libfs_mgr to reduce code
duplication (in terms of source code and code gen)

Note:
init's implementation checks the kernel cmdline first and then the
kernel bootconfig, while libfs_mgr's order is the opposite.
Realistically I don't think this order matter much though. If any, we
should prioritize bootconfig over kernel cmdline most of the time.

Bug: 293695109
Test: Presubmit
Change-Id: Ic8d2c965c62f9e873ccdaf77d67c7708f25a7b56
(cherry picked from https://android-review.googlesource.com/q/commit:d7c67b40a9b6a5e72d54adf37da22238381182f7)
Ignore-AOSP-First: Fix merge conflict