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
