aconfig_storage_read_api: allow read api to read from mutable mapping

In aconfigd, we also need to read from mutable storage files (persist
copies of storage files). Current read API only accepts Mmap which is
immutable. We need it to also accept MmapMut. Since both Mmap and
MmapMut deref to &[u8], change the function signature to accept &[u8].

Bug: b/327170179
Test: atest -c
Change-Id: I0a7d99bc38176c2aeae8c20f749419caf1d365fa