[commands] Fix JoystickButton.getAsBoolean (#4131)

This previously always returned false; the get method it inherited was not used in the getAsBoolean defined in the Trigger class. The fix is to swap get() and getAsBoolean() implementations in the Trigger class.

wpilibNewCommands/src/test/java/edu/wpi/first/wpilibj2/command/button/ButtonTest.java

@@ -195,4 +195,13 @@ class ButtonTest extends CommandTestBase {
     scheduler.run();
     verify(command).schedule(true);
   }
+
+  @Test
+  void booleanSupplierTest() {
+    InternalButton button = new InternalButton();
+
+    assertFalse(button.getAsBoolean());
+    button.setPressed(true);
+    assertTrue(button.getAsBoolean());
+  }
 }