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/tests/check_prop_prefix.py b/tests/check_prop_prefix.py
index 68511ce..13a7b99 100644
--- a/tests/check_prop_prefix.py
+++ b/tests/check_prop_prefix.py
@@ -76,7 +76,7 @@
     print('%d violations found:' % len(violations))
     print('\n'.join(violations))
     print('******************************')
-    print('%s contains properties which are not properly namespaced.' % args.property_contexts)
+    print("vendor's and odm's property_contexts MUST use ONLY vendor-prefixed properties.")
     print('This is enforced by VTS, so please fix such offending properties.')
     if args.allowed_property_prefix:
         print('Allowed property prefixes for %s: %s' % (args.property_contexts, args.allowed_property_prefix))