storage daemon: update bytes format sent over socket

Before this cl, the bytes sent over this socket is just (proto), now it
will be (proto byte size | proto). So both sender and reciever can
allocate proper buffer without using a fixed size buffer.

Bug: b/312444587
Test: atest aconfigd_test
Flag: enable_aconfig_new_storage
Change-Id: I3737f9ecfaf684610a1c4307ae7b6f0ce1f515bc
6 files changed