init.rc: create /metadata/aconfig, /metadata/aconfig/flags and

/metadta/aconfig/boot dirs

Create aconfig dirs on /metadata to store aconfig storage related files.

Under /metadata/aconfig we will store the following pb files:

1, aconfig_storage_location.pb, store the location of the storage files for
each container.

2, aconfig_flag_persistent_overrides.pb, store the local flag value overrides.

Under /metadata/aconfig/flags we will store flag value and info files
for each container, this include:

1, <container_name>.val flag value file
2, <container_name>.info flag info file

Under /metadata/aconfig/boot we will store read only flag value files
copied from /metadata/aconfig/flags at boot. These read only files are
used to serve flag read queries.

Bug: 312444587
Test: m
Change-Id: I8ae06e56fc9b9e8c0b06f86e3deb4219d7f49660