Fixed deadlock in ShadowLooper.

Deadlock happaned whne `HandlerThread` was started and main thread was callin
`Looper.getMainLooper()`.

Can be reproduced with next code:

  @Test
  public void testRobolectricHang() {
    for (int i = 0; i < 100; i++) {
      HandlerThread thread = new HandlerThread("");
      thread.start();
    }

    long end = System.currentTimeMillis() + 5000;
    while (end > System.currentTimeMillis()) {
      Looper.getMainLooper();
    }
  }

This diff fixes the lock by not holding the lock on the ShadowLooper.class and
using a static Object instance for synchronization.