pom2mk: Add a flag to specify a specific version of artifacts

For repositories that contain multiple versions of the same artifact,
we'd only like to create makefiles for one of the versions of the
artifact.

Bug: 64723465
Test: cd prebuilts/maven_repo/android; pom2mk -use-version 25.3.1 com/android/support
Change-Id: I5bdbba8d2143a47610d56d679381eb01b3cf176d