libfs_avb: refactoring

This commit has the following changes:

    1. Builds libfs_avb via "fs_mgr/libfs_avb/Android.bp" instead of
       "fs_mgr/Android.bp", and removes the libfs_avb source
       dependencies on "fs_mgr/fs_mgr_priv.h".

    2. Moves static functions in fs_avb.cpp into util.cpp or
       avb_util.cpp, depending on whether the function is related to
       AVB or not.

    3. Introduces two host unit tests: libfs_avb_test and
       libfs_avb_internal_test, the former is to test public
       <fs_avb/fs_avb.h> APIs, while the latter is to test libfs_avb
       internal functions.

    4. Splits fs_avb_unittest_util.* into:
       - fs_avb_test_util.* (host static lib: libfs_avb_test_util),
       - basic_test.cpp (host executable: libfs_avb_test)

Bug: 112103720
Bug: 117960205
Test: atest libfs_avb_test
Test: atest libfs_avb_internal_test
Test: boot a device
Change-Id: I11d6c9e9019e20b594d9321b9a28118d4806e5a7