apex: dedup jni_libs when generating apex_manifest

When we package both 32/62, there could be two instances of the same
library. We need to dedup the list of jni_libs.

Bug: n/a
Test: m com.android.cronet (which uses jni_libs)
      check apex_manifest.pb
Change-Id: I9a9f42f0f5219ab7bc50d945b54113b5d823b7f3