demux: improve replaygain tag handling

Refer to vlc-android#2907 for details.

The sample files contain lowercase tags:

replaygain_track_gain: -4.75 dB
replaygain_track_peak: 1.000359

Modern tools standardized on uppercase tags by default:

REPLAYGAIN_ALBUM_GAIN: -6.25 dB
REPLAYGAIN_ALBUM_PEAK: 1.000000
REPLAYGAIN_TRACK_GAIN: -4.73 dB
REPLAYGAIN_TRACK_PEAK: 1.000000