Move HIDDENAPI_FLAGS dependencies to frameworks/base

Recent move of hiddenapi rules to soong sets the dependencies of
the hiddenapi_flags.csv rule in dex_preopt.mk but doesn't check
for existence of "frameworks/base" as the hiddenapi-generate-csv
rule in frameworks/base. Solve this by removing the dependency
injection and using the SOONG_HIDDENAPI_FLAGS variable directly
in frameworks/base.

This is necessary because go/art-build creates a fake
hiddenapi_flags.csv and if there is a dependency injection rule,
a phony target is created and all dependencies of flags.csv need
to be satisfied.

Test: master-art compiles
Change-Id: I13ef5e7ec167f9372a5629fe1894c7acd4091d29