Fix flaky navBar test in CTS

This patch is to fix the timing issue, which is adding a navigation bar when
a new display with system decoration support is created. When WMS#hasNavigationBar
is called in createNavigationBar, the method may return false even if the display
DOES support system decorations since DisplayContent is not yet created.

It is because RootActivityContainer, which creates ActivityDisplay and also DisplayContent
indirectly, uses DisplayThread to do it, which makes itself not receive the message
until everyone is informed about the change. Change to use UiThread to fix
this issue.

Bug: 120748674
Test: atest ActivityMultiDisplayTests
Test: atest ActivityMultiDisplayTests#testNavBarShowingOnDisplayWithDecor * 200
Change-Id: I733bbbe8deee452af87f0f1ca09316c207d82da3