Implement AImageDecoder_setInternallyHandleDisposePrevious

Bug: 160984428
Test: I00682f201a52f894b0e1335c00c4368ce675a805

Also fix a bug caught by the new test. If the current frame is the first
in a series of one or more RestorePrevious frames, fPriorFrame should be
set to |currentFrame - 1|. Otherwise SkCodec will decode the required
frame. This is wasted work, since the prior frame should already be
prepared (either by AImageDecoder or by the client).

Change-Id: I1fb9f91dc66fd3121f187b9a91c15f625eb17f8d