Modifty the documentation for MediaRouter2.showSystemOutputSwitcher

- Ignore calls to showSystemOutputSwitcher for apps not in the
foreground.
- Change method signature to return boolean flag indicating if dialog
was shown or not.

Bug: 266755845
Test: Using repeated calls in Sample App.
Change-Id: Ief3b9d3c7e0f340c5cebf906a880d5f7eba71dbf