Fix proguard_usage.zip with USE_RBE=true RBE_R8=true

proguard_usage.zip contained an empty unused.txt file when
the unused.txt file wasn't specified as an output of the r8
rule.

Test: m USE_RBE=true RBE_R8=true out/soong/.intermediates/frameworks/base/packages/Shell/Shell/android_common/proguard_usage.zip
Change-Id: Ie6a7c8b2581fe21d643e675143d6cc0e2ce80456