MIME types are case-insensitive, but none of the many existing
comparisons across our code base take this into account. The
code can be made more robust by normalizing all MIME types at the
moment they are first set into a class/builder and adding toLowerCase
as part of the normalization.
Most concretely, this fixes an issue with playing HLS streams via
the IMA SDK where the stream MIME type is indicated with all lower
case "application/x-mpegurl", which failed the MIME type comparison
in DefaultMediaSourceFactory.
PiperOrigin-RevId: 582317261
b570c72588