Add global enforcing test.

SELinux tests are primarily enforced via host-side tests to take
advantage of privileges available to the shell user that are not
available to cts.  Add a check that explicitly checks to make sure
SELinux policy is being enforced on the system, rather than relying on
results from various checkAccess() tests in SELinuxHostTest.

(cherry-pick of internal commit: f4c87b375f2a449978ba0774098075f2aa069aa1)

Bug: 27756382
Bug: 27763829
Change-Id: I369e7ab78a41718c464a3fd70163ba398a040730
1 file changed