Replace browser.rootEl with browser.setAngularRoot()

[heathkit]

Similar to replacing browser.ignoreSynchronization with browser.waitForAngularEnabled(), we should change setting the root element from a property to a method call.

[heathkit]

Also, make sure we do the right thing in this case:

browser.setAngularRoot('x');
browser.get('http://something.com').then(() => {
   element(by.css('button')).click();
});
browser.setAngularRoot('y');
element(by.css('other_button')).click();

[sjelin]

Same with useAllAngularTestabilities