Validate no-cross-domain /proc/PID access

Android uses hidepid=2 to restrict visibility to other /proc entries on
the system. This helps preserve user, application, and system
confidentiality by preventing unauthorized access to application metadata,
and addresses attacks such as
http://www.cs.ucr.edu/~zhiyunq/pub/sec14_android_activity_inference.pdf

Ensure the SELinux (weaker) equivalent is being enforced by adding
neverallow compile time assertions.

TODO: The "shell" user runs as both an Android application, as well as
spawned via adb shell. This was a mistake. We should separate out the
"shell" Android app into it's own SELinux domain. For now, exclude the
shell from this assertion. (The shell Android app is covered by
hidepid=2, so there's no leaking of data, but still, it's over
privileged today and should be cleaned up.

Bug: 23310674
Test: policy compiles. Compile time assertion only.
Change-Id: I0e1a6506b2719aabf7eb8127f046c4ada947ba90