Improve android_app_import.dpi_variants handling.

Instead of circumventing the limitation of Prebuilt implementation by
picking a source path itself, it now uses the same mechanism as
archMutator and replaces the source path in advance so that Prebuilt
always sees the corrent source path.

Because this requires the Apk field to be a string pointer, the single
source prebuilt implementation is being updated to be reflection-based.

Test: Soong unit tests, m soong_docs, TreeHugger
Change-Id: I2304f15e32d632f74f95f0d9e9bf1f75ff3e2225