Validate monolithic and modular hidden API flags are consistent

This makes sure that where there is overlap between the hidden API
flags generated for a module and the monolithic flags that they are
identical. That ensures that the modular hidden API flags will be
compatible with previous releases that relied on the monolithic flags.

Bug: 179354495
Test: m out/soong/.intermediates/art/build/boot/art-bootclasspath-fragment/android_common_apex10000/modular-hiddenapi/all-flags.csv
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - Create some inconsistencies between the above two files.
      m out/soong/hiddenapi/hiddenapi-flags.csv.valid
Change-Id: Iaf9e23cef63e221608955d89dc8d496bcc70c86e