HeartRateMonitor upgrade to unified

Removed old “MonoMac” namespace qualifier from several types. Changed
int to nint, and RectangleF to CGRect.

Changed second parameter in call to NSTimer.CreateScheduledTimer:
NSTimer.CreateScheduledTimer (60 / (double)CurrentHeartBeat.Rate, new
Action<NSTimer>((x)=>ScheduleBeatTimer()));

Changed CoreBluetooth.PeripheralConnectionOptions member from
NotifyOnDisconnectionKey to NotifyOnDisconnection.

Added “com.xamarin.heartratemonitor” to info.plist.