storage daemon: handle flag info file update

1, when the persistent flag value is updated, mark in flag info file
that this flag has override.

2, when querying a persistent flag, also return the flag info along with
flag value.

Bug: b/312444587
Test: atest aconfigd_test
Flag: enable_aconfig_storage_daemon
Change-Id: I98efe114013e775b853cbfe6e5dba8d6212a3f20
7 files changed