Fix ListMixinTest and RecyclerMixinTest for KitKat

A dexmaker issue with KitKat makes it so that accessing package
private methods on a spy will throw IllegalAccessError. Use mock with
delegatesTo instead to workaround the issue.

Test: ./gradlew connectedAndroidTest
Change-Id: I18ad132593a646e6955024caaef0238486b69703