Allow specifying the manifest path and resource path via a system property

gradle-android-test-plugin puts following paths to system properties.
 - manifest 
 - resources
 - assets

see https://github.com/novoda/gradle-android-test-plugin/blob/master/gradle-android-test-plugin/src/main/groovy/com/novoda/gradle/test/VariationConfigurator.groovy#L73