Add a preprocessed_ndk_headers module type.

Unlike the old version of this (which has been renamed to
verioned_ndk_headers), this module includes a property to specify the
preprocessor to be used.

This is going to be used for preprocessing ICU4C's unicode/uconfig.h
to alter a config value that needs to differ between the platform and
the NDK.

Test: make checkbuild
Bug: https://github.com/android-ndk/ndk/issues/548
Change-Id: If5c6f54f90cf5a4187693a941748d72f39b0d797