Add AImageDecoder _advanceFrame and _rewind

Bug: 160984428
Test: Iae7d274b69999c471fd5610c6ef4d148cca81bec

These enable decoding all frames of an animated image (e.g. GIF).

Also add ANDROID_IMAGE_DECODER_FINISHED, which can be returned by
AImageDecoder_decodeImage or AImageDecoder_advanceFrame when the end of
the animation has been reached.

Disallow AImageDecoder_set* methods after the first frame, since
changing the settings would interfere with blending and caching for
kRestorePrevious frames.

Specify that a single frame GIF is not animated.

Change-Id: I6a0738d2e8ed27062c86d7b6dce37d54425552f2