Fix some bugs in allowing to start/and/stop the handler
Fix some bugs in allowing to start/and/stop the handler

 * If at full volume while initializing, the system HUD would sometimes
incorrectly display the volume initialization back down to 0.99
 * In some cases (seemed to repro consistently on later iOS versions), the
volume was being incorrectly reset to either 0 or 0.5

jpsim/JPSVolumeButtonHandler