Ensure only com.android.shell can run in the shell domain.

Don't allow apps to run with uid=shell or selinux domain=shell unless
the package is com.android.shell.

Add a neverallow assertion (compile time assertion + CTS test) to ensure
no regressions.

Bug: 68032516
Test: policy compiles, device boots, and no obvious problems.
Change-Id: Ic6600fa5608bfbdd41ff53840d904f97d17d6731