activity_main: Add a new button

Add an stop button and place it on the right side of the stop button.
strings: Update some strings and colors

Add colors for the diffrent states. Add a string for the stop button and update
the default string of the timerView.
MainActivity: Add timer functionality

Add the timer which runs in an extra thread. Add OnClickListeners for both
buttons which starts and stops the timer.
Removed unused class files