Fix java_import and android_library_import conversions

java_import and android_library_import modules can't be handled
directly in androidmk because the results may depend on properties
that haven't been parsed yet.  Add a bpfix pass (which is
automatically included at the end of androidmk) to select
android_library_import vs. java_import based on the extension
of the prebuilt file, and convert the srcs property to jars or aars
as appropriate.

Bug: 73724997
Test: androidmk_test.go
Change-Id: I1024742e9e96d5e1e88c3cc139eeb0d5a2f6849b