Add rules for dex2oat.

This is necessary (but not sufficient) to allow CompOsTestCase to
run. Without it we have problems because
system/sepolicy/apex/com.android.art-file_contexts references
dex2oat_exec, and we get an error:

SELinux : Context u:object_r:dex2oat_exec:s0 is not valid (left unmapped).

Bug: 194474784
Test: atest CompOsTestCase (with memory fix patched in)
Change-Id: If229b9891c8fb1acce8d0502675c1712bfed180c

microdroid/system/private/dex2oat.te

@@ -0,0 +1,5 @@
+# dex2oat
+type dex2oat, domain, coredomain;
+type dex2oat_exec, system_file_type, exec_type, file_type;
+
+allow dex2oat tmpfs:file { read getattr map };