Allowing the fs_mgr flag of avb_keys to be a dir

Hard coding multiple keys in the avb_keys flag isn't flexible and
causes some pains when upgrading an Android codebase.
e.g., from Android 10 to Android 11.

This CL supports specifying a directory for the avb_keys
for fs_mgr to list then use the avb keys under the directory.

Bug: 144399552
Test: config a fstab using avb_keys as a dir to boot
Test: atest libfs_avb_test
Test: atest libfs_avb_internal_test
Change-Id: Ie74845d8c8e4aa45e8a9e3b862424cec641f8090