Add "shell" to seapp_contexts

In the process of taking a bugreport, "dumpstate -B" will instruct
zygote to fire up com.android.shell, which runs as UID=2000.
This transition was not included in seapp_contexts, so zygote
didn't know how to properly set the context for the shell user.

Add an entry to allow zygote to know what to do with UID=2000
requests.

Bug: 9588981
Change-Id: I2e726be8d58437ef1de3bcbad3b897a97ed18e22