The new class EmailAsyncTask might look overkill, but
this is what I've been wanting for long time.
In many activities we store all AsyncTasks we start to member fields
so that we can cancel them in onDestroy(). (e.g.
MessageViewFragmentBase.mLoadMessageTask and mReloadMessageTask)
With EmailAsyncTask these fields will no longer be necessary.
We'll be able to just fire up as many AsyncTasks as we want, and clean them
up in onDestroy() with just cancellAllInterrupt().
Bug 3480136
Backport of Id8aa1ba1500eee58cfab8b562b95e9ed852b3e29
Change-Id: I2d2966ff878862a5246c031d1d4e221da5a7e81a
ef2bc0b3e9