Enable StreamLoader to be loaded in a separate thread.

Move ContentLoader and FileLoader to this new way
as they involves IO. Will work on CacheLoader later.

Change StreamLoader to contain a Handler instead of
derive from a Handler so that the Handler can be
created in the thread where load() is called.

Rename StreamLoader's old "LoadListener mHandler"
to mLoadListener.

Remove unused import and unreachable exception.

Fix http://b/issue?id=2158613

This improved page_cycler performance in moz/intl by
10-30% as we are not blocked by IO any more.