Add AVIF mime type to android.mime.types

Associate "avif" extension to "image/avif" which is the mime type
for AVIF images. Specification:
https://aomediacodec.github.io/av1-avif/#mime-registration

Unlike HEIF, AVIF does not have separate mime types for still images
and sequences:  https://github.com/AOMediaCodec/av1-avif/issues/59

Bug: 141654151
Change-Id: I34711d5354aac0b78e89502f84debf9ce0a1ddff