Changed Android Maven plugin version to 3.9.0

Build fails because aapt tool has been put in a different folder. This is fixed in version 3.7.0 as per https://code.google.com/p/maven-android-plugin/issues/detail?id=377&sort=-id&colspec=ID%20Type%20Component%20OpSys%20Status%20Priority%20Milestone%20Owner%20Summary