Stop using WeakReference for bitmaps to be published.

To conserve memory, previously we used WeakReferences to store Bitmap
references for cards to be published. In slow network situations, these
could be garbage collected before the publish occurred. Change these to
strong references instead and recycle them as soon as we can.

Change-Id: I0bf9c32b7a34e345e2d2cd3312375fa78b47e78c