Fix how ExifInterface creates VP8X chunks for WebP files

The current code reads alpha into an int, doesn't shift it back to the
least-significant-bit, and then shifts that value 4 more bits before
writing to the VP8X 'flags' byte. This results in setting the MSB of the
flags byte, which is a reserved bit that must always be zero, resulting
in an invalid file.

VP8X part of WebP spec:
https://developers.google.com/speed/webp/docs/riff_container#extended_file_format

Test: ExifInterfaceTest (after removing the suppression for this bug)

Bug: 253622642
Change-Id: I42469a479c94442eb395124c4eac174bd9ab6e09