Do not package res unless RS target API < 21

Bug: 37626838

Since Lollipop, RS generated bitcode is embedded as strings in
generated Java files, and no longer needed in the resources.
This CL makes packaging resources conditional on RS target API, and
not do so unless it is below 21.

Test: mm in frameworks/rs/tests/java_api/Refocus, ImageProcessing2, and
      ImageProcessing_jb
Change-Id: I79a90ed4b96cb78b22a64a35b539d9d67351f4c4