Replace epoll() with poll() and rename PollLoop to Looper.

As part of this change, consolidated and cleaned up the Looper API so
that there are fewer distinctions between the NDK and non-NDK declarations
(no need for two callback types, etc.).

Removed the dependence on specific constants from sys/poll.h such as
POLLIN.  Instead looper.h defines events like LOOPER_EVENT_INPUT for
the events that it supports.  That should help make any future
under-the-hood implementation changes easier.

Fixed a couple of compiler warnings along the way.

Change-Id: I449a7ec780bf061bdd325452f823673e2b39b6ae