Introduce display windowing mode to WindowConfiguration

Introduce mDisplayWindowingMode to WindowConfiguration, and show caption
for standard activities if the display windowing mode is freeform.

Test: Mannual. On secondary display, full screen activities will have a
caption bar, e.g., Pixel Logger.
Test: go/wm-smoke
Test: atest DisplayContentTests
Test: atest WindowConfigurationTests
Test: atest ConfigurationContainerTests

Change-Id: Iedaeb0886bd6d61ad29db0ba98def7c004298afd