Let APEX build against preferred java_sdk_library_import

If a java_sdk_library_import has a corresponding java_sdk_library then
make the java_sdk_library_import export both the xml and impl modules
created by the java_sdk_library.

Makes java_sdk_library_import consistent with java_sdk_library by
providing libraries within the same APEX access to the implementation
library instead of the stubs.

Bug: 158304459
Test: m nothing
      Test what happens when building com.android.tethering with and without
      a preferred "framework-tethering" java_sdk_library_import. Make sure that
      it does not change the generated APEX.
Change-Id: I2f4edea937ac377431a5696c92cbd467bded62ef