Decoder: Treat first slice in a picture as part of new picture always

This is needed to decode streams with consecutive IDRs.

Bug: 34097231
Test: successful run of POC in security bug
Change-Id: Ib737a4ef4b8c5bb7a57c90292102dd28af0615fe
(cherry picked from commit 8b5fd8f24eba5dd19ab2f80ea11a9125aa882ae2)
1 file changed