Write a scheduler based on the RxJava implementation

This scheduler is more efficient *and* doesn’t require an Activity
context
Remove automatic setup of AndroidUIScheduler
Set the scheduler on startup on Android
Remove DeviceScheduler

DeviceScheduler has some bad caveats, use the native schedulers instead
Merge pull request #717 from reactiveui/android-scheduler-fixes

Android scheduler fixes
Mark Dispose as virtual, fixes #709