Introduce ParallelExecutorCompat into support/v4

This class provides a backward compatible access to
a shared instance of ThreadPoolExecutor.

Also using this class in AsyncListUtil implementation
to avoid creating a new single thread executor per
instance.

Fixed tests accordingly.

Bug: 22640199
Change-Id: I89a59854f436c7dd9a8a9ab30fc86bd8f50e9df7
8 files changed