Fix burst of Game.Update calls at the start of the game

Found while investigating #2595
Test project: https://github.com/danzel/DrawTextTest/tree/isrunningslowlytest

We start the Game._gameTimer Stopwatch when the Game class is constructed. This is before we have done any of the system initialisation, which can take a while.
As such, the first time we get in to Tick, we think the game has already been running for quite a while and we need to do lots of Update calls to catch up (usually _maxElapsedTime  worth).

Instead now we only start the StopWatch when we are about to start the Update/Draw loop.

With the above test project on XNA start up looks like (see out.txt):
```
Update 0 False, 
Update 1 False, 
Draw 0 False, 
Update 2 False, 
Update 3 False, 
Draw 1 False, 
Update 4 False, 
Draw 2 False, 
Update 5 False, 
(And so on, Update Draw...)
```

But in Monogame it looked like:
```
Update 0 True, 
Update 1 True, 
Update 2 True, 
Update 3 True, 
Update 4 True, 
Update 5 True, 
Update 6 True, 
Update 7 True, 
Update 8 True, 
Update 9 True, 
Update 10 True, 
Update 11 True, 
Update 12 True, 
Update 13 True, 
Update 14 True, 
Draw 0 True, 
Update 15 True, 
Update 16 True, 
Update 17 True, 
Draw 1 True, 
Update 18 False, 
Draw 2 False, 
Update 19 False, 
Draw 3 False, 
(And so on)
```

Now the start of monogame looks like:
```
Update 0 False, 
Draw 0 False, 
Update 1 False, 
Update 2 False, 
Draw 1 False, 
Update 3 False, 
Draw 2 False, 
Update 4 False, 
Draw 3 False, 
(silky smooth!)
```