The UserInterface can only be updated by the UI Thread. You need a Handler, to post to the UI Thread:
private void startTimerThread() {
Handler handler = new Handler();
Runnable runnable = new Runnable() {
private long startTime = System.currentTimeMillis();
public void run() {
while (gameState == GameState.Playing) {
try {
Thread.sleep(1000);
}
catch (InterruptedException e) {
e.printStackTrace();
}
handler.post(new Runnable(){
public void run() {
tvTime.setText("" + ((System.currentTimeMillis() - this.startTime) / 1000));
}
});
}
}
};
new Thread(runnable).start();
}