Add aconfig flag support for android_app

This change adds an overrideable property flags_packages to android_app,
which is used to list the aconfig_declarations module names that the app
depends on. The build action of android_app is modified to pass all
flags text file provided by the aconfig_declarations to aapt2 link as
--feature-flags arguments.

Test: m nothing --no-skip-soong-tests
Bug: 306024510
Change-Id: I4924f88b9954950cc1936a472cd7ac70f41add5d