Ignore inPurgeable and inInputShareable.

These are discouraged anyway, due to the fact that using them may
result in decoding on the UI thread. Now that they are ignored, the
images will still be decoded; they just will never have their
memory purged, meaning there could be a RAM penalty. This is
acceptable, since apps are encouraged to instead use inBitmap to
manage bitmap memory.

This incidentally fixes BUG:15390468 by converting purgeable index8
(e.g. gif) images to non purgeable.

Change-Id: Ieaf3ab25d28d93fb94bdaea1eb3bd762f163b09a