Clear Background flag when thread is stopped

[MichalStrehovsky]

System.Threading.Thread.Tests

[FAIL] System.Threading.Threads.Tests.ThreadTests.ThreadStateTest
Assert.Equal() Failure
Expected: Stopped
Actual:   Background | Stopped
   at System.Threading.Threads.Tests.ThreadTests.ThreadStateTest() + 0x204
   at System.Threading.Thread.Tests!<BaseAddress>+0x884418
   at System.InvokeUtils.CallDynamicInvokeMethod(Object, IntPtr, IntPtr, IntPtr, Object, Object[], BinderBundle, Boolean, Boolean) + 0x13c