Add new snapuserd socket and property rules.

This adds a new property prefix owned by snapuserd, for communicating
when the service is ready to accept connections (snapuserd.ready and
snapuserd.proxy_ready).

This also adds a new socket context. This is a seqpacket socket used to
communicate with a special instance of snapuserd that bridges to the
first-stage daemon.

Bug: 193833730
Test: no denials after OTA applies and boots
Change-Id: Ibad03659eba5c25e205ba00f27d0b4f98585a84b
6 files changed