runas: Grant access to seapp_contexts_file

Runas/libselinux needs access to seapp_contexts_file to determine
transitions into app domains.

Addresses:
avc: denied { read } for pid=7154 comm="run-as" name="plat_seapp_contexts"
dev="rootfs" ino=9827 scontext=u:r:runas:s0
tcontext=u:object_r:seapp_contexts_file:s0 tclass=file

Bug: 36782586
Test: Marlin policy builds
Change-Id: I0f0e937e56721d458e250d48ce62f80e3694900f

public/runas.te

@@ -23,6 +23,10 @@ selinux_check_context(runas) # validate context
 allow runas self:process setcurrent;
 allow runas non_system_app_set:process dyntransition; # setcon
 
+# runas/libselinux needs access to seapp_contexts_file to
+# determine which domain to transition to.
+allow runas seapp_contexts_file:file r_file_perms;
+
 ###
 ### neverallow rules
 ###