Make the runas domain enforcing.

Change-Id: I4b3dda1d08e8bfc523493f4b8d79a4cc3e7e7787
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

runas.te

 type runas, domain, mlstrustedsubject;
-permissive runas;
 type runas_exec, exec_type, file_type;
 
 # ndk-gdb invokes adb shell run-as.