Stability Improvements

- Simplified _hiraganaToKatakana and _katakanaToHiragana to reduce
redundancy and prevent a lot of "OutOfIndex" exceptions
- _isChar functions now ask for a char to reduce number of calls to
String.valueOf()
- _hiraganaToRomaji no longer crashes when passing in all-romaji
- _romajiToKana now returns ッ instead of a っ if it detects
double-uppercase-consonants
- Removed unnecessary imports from the MainActivity class