Make CombinedMessageQueue pick an implementation per instance

This makes the class play more nicely with testing solutions
such as TestLooper and Robolectric Shadows that use
reflection to drive MessageQueue.

Flag: build.RELEASE_PACKAGE_MESSAGEQUEUE_IMPLEMENTATION
Change-Id: If0aaa8d5e57aad1e7200e8900ca9fc5b35c8e0b5