aconfig: add support for local override

1, add a new field in storage metadata proto, for each container, there
is a local override file.

2, update write api to expose the api to map a writable file given the
specific path. the cannonical api will require a storage metadata proto,
and then find the right file path and map it.

3, minor update to make the proto lib cc_library instead of
cc_library_static

Bug: b/312444587
Test: atest -c
Change-Id: Iaf0aff44c1ca3ad4bffc5e06bb99bb276b9069c5