fs_mgr: fix fs_mgr avb_keys parsing problem

We should check if the fs_mgr option starts with "avb_keys" before
"avb". Otherwise, it will treat "avb_keys" as "avb" fs_mgr option.

Bug: 112103720
Test: atest fs_mgr_unit_test

Change-Id: I88446222fa88e8ecfcd6f96d30ad4336ebe146a8