Add macros to flag-guard te and contexts files

This adds two macros which can be used in te files and contexts files.

* is_flag_enabled(flag_name, codes)
* is_flag_disabled(flag_name, codes)

Also flag-guarding requires to process input files before any
validations. Property contexts test and seapp contexts test are
modified a little to handle that.

Bug: 306563735
Test: build with manual guarding
Change-Id: Ia1c6d00b7aab0da3901c19f16d553153aace018c
diff --git a/compat/Android.bp b/compat/Android.bp
index 9768eb1..2c6239f 100644
--- a/compat/Android.bp
+++ b/compat/Android.bp
@@ -429,6 +429,7 @@
 
 se_compat_test {
     name: "sepolicy_compat_test",
+    defaults: ["se_policy_conf_flags_defaults"],
 }
 
 se_build_files {