MPVActivity: Set initial brightness for touch gestures correctly
MPVActivity: Set initial brightness for touch gestures correctly

This makes brightness adjustment consistent with seeking/volume,
that is adjustments are made relative to the current value.

jeeb/mpv-android,jeeb/mpv-android,mpv-android/mpv-android,mpv-android/mpv-android,mpv-android/mpv-android,mpv-android/mpv-android,jeeb/mpv-android,jeeb/mpv-android,mpv-android/mpv-android,jeeb/mpv-android