heif: allow files without valid image sequence track to extract

Some heif files advertise 'hevc' brand but don't have valid
image sequence tracks. These are technically non-compliant,
but allow the still images to extract as long as some still
images are found.

Also fix a type in the format conversion code.

bug: 72363209
bug: 63633199
Change-Id: I5b6c486cdccb06c6d45090967650791eb9b513cf