Merge libs/ directory of imported aars into classes.jar

AARs can provide extra classes in the libs/ directory that should
be treated the same as classes.jar, merge them into a single jar
when extracting the AAR.

Bug: 162892721
Test: m androidx.appsearch_appsearch
Change-Id: I92bf46f47ce5f3dce16c42197c15ed256304fda2