UI and functional changes
MainActivity:
-displays separate error if no calendars have been selected to be synced
-updated colors used for ringer icons:
    default - light grey
    calendar - blue grey
    event series - green grey
    event instance - dark grey

SelectCalendarsActivity:
-redesigned list layout (text is still overlapping icons on right side if the calendar name is too long)
-clear button is only visible if the calendar has a previous setting applied
-checkbox used instead of changing the list items background color

SettingsActivity:
-removed sliders for media and alarm volumes, toggling either checkbox will now totally silence the media and/or alarm volumes during an event

 VolumesManager:
 -current media and alarm volumes are stored in preferences before the channels are silenced and are restored after the event ends

 Prefs:
 -removed methods related to the variable alarm and media volumes
 -added methods to store and retrieve media and alarm volumes