Remove Handler usage from AndroidAsync.

This belongs in a higher level library, like ion.

Change-Id: Ia5a73d7d3f6165a9868b0720265ad08f6cf8c00b
Performance fixes around bitmap load cancellations.

Previous implementation was scorching LoadBitmap from the pending map even if it is in progress.
Losing that history caused more than MAX_IMAGEVIEW_LOAD concurrent downloads.

Change-Id: I819de8218bdb617c3f07ccdccd5b34b142387447