Disable Proguard for cts tests.

Previously the build system would include
build/core/proguard_tests.flags, which virutally disables Proguard,
if an app links against android.test.runner.
Now you have to disable it explicitly.

Bug: 10307372
Change-Id: I78ea75ef4770f4a4b761b9b9996e745f417646cc