annogen: make --delete-existing-ruby in conjunction with --sharp-multi and existing-ruby options make sense

annogen Android apps: mode selector for sharpMulti + fix what happens if GOOGLE_PLAY_TRACK is set to empty string

Remove deprecated iOS browser app: the App Store stopped accepting UIWebView apps in April (updates are allowed till December but I think all existing annogen-influenced iOS apps use JS/Dart instead)

annogen Android apps: try to avoid other apps intercepting EPUB URL when we want it downloaded for annotation