fix: crash when adding or editing too many multi-instruments in the menu (stack overflow because of opening the main multi-instruments menu, now it just goes back and there's no infinite PreferenceScreen history!)

Signed-off-by: Stephen L. <lrq3000@gmail.com>
update: trying to keep SoundPool cached sounds in memory (but doesn't quite work...)

Signed-off-by: Stephen L. <lrq3000@gmail.com>