Skip Proguard for EXPERIMENTAL_USE_OPENJDK9=true

Proguard doesn't support v53 class files. For now, this
CL simply replaces the Proguard step with a plain 'cp',
which is essentially a no-op.

To avoid apps exceeding the dex limit in this mode, this
CL also enables --multi-dex for those builds.

Test: Treehugger
Bug: 67673860
Change-Id: I17dc5d827360d606a0cac12e324f97e8dedbb630