Ensure Android callback handler always on main thread

Since the contract of AndroidDeferredManager is that the callbacks
always occur on the main thread, even if the AndroidDeferredObject is
instantiated on a background thread.
Merge pull request #32 from jblebrun/android_handler_always_main

Ensure Android callback handler always on main thread
Added missing import.
[maven-release-plugin] prepare release 1.2.2
[maven-release-plugin] prepare for next development iteration