Special-case java_sdk_library in source vs prebuilt selection

If a java_sdk_library is listed in `contents`, then restrict the source
vs prebuilt selection superseding to only the internal stub libraries.

Defer the source vs prebuilt selection mechanism of the top-level
library to the existing mechanisms for now. This is necessary because
this top-level library acts as a hook for
- hiddenapi: boot jars are deapxed from the prebuilt apex
- dexpreopt: system server jars are deapexed from the prebuilt apex and
  installed via required

If `next` uses `framework-foo` and `service-foo` as the top-level
library instead of the prebuilt equivalents, then the bootjars installed
in out/soong/dexpreopt_x86_64/apex_bootjars will come from source. And
the *.odex files of the system server jars will come from source.

Bug: 308174768
Test: Added a java_sdk_library unit test to assert that the new
mechanism supersedes the `prefer` flag

Change-Id: Ib43198a3b547c58b54f1f0966e95584215096d32