Add DisplayContent#forceDesktopMode

As WMS#mForceDesktopModeOnExternalDisplays is a flag to force enable
desktop mode on public secondary display.

Since some code points didn't check if the display is public
& secondary display, it may cause some issues like Bubbles using
private virtual display but the content can show system decorations and
IME always failed to show, if user enabled force desktop mode config
from developer option.

Add DC#forceDesktopMode to make sure the check is proper and consistent.

Fix: 163333691
Test: manual as test steps:
1) Enable "Force Desktop Mode on External display" from Settings ->
   Developer option.
2) Launch Message or FB messenger with chat head bubbles
3) Taping EditText and check if soft-keyboard can show up correctly.
Test: atest DisplayContentTests#testForceDesktopMode

Merged-In: I6ec52b6b1642fd547ebc222ae1bf9a2958e5fb2d
Change-Id: I6ec52b6b1642fd547ebc222ae1bf9a2958e5fb2d