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
diff --git a/private/file_contexts b/private/file_contexts
index c9b7c69..a5dd5a6 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -162,6 +162,7 @@
 /dev/socket/rild	u:object_r:rild_socket:s0
 /dev/socket/rild-debug	u:object_r:rild_debug_socket:s0
 /dev/socket/snapuserd u:object_r:snapuserd_socket:s0
+/dev/socket/snapuserd_proxy u:object_r:snapuserd_proxy_socket:s0
 /dev/socket/tombstoned_crash u:object_r:tombstoned_crash_socket:s0
 /dev/socket/tombstoned_java_trace u:object_r:tombstoned_java_trace_socket:s0
 /dev/socket/tombstoned_intercept u:object_r:tombstoned_intercept_socket:s0