Introduce IWindowManager#isEligibleForDesktopMode()

The difference of this API and existing shouldShowSystemDecors(displayId) is:

- shouldShowSystemDecors(displayId) returns true for all the displays
  that should show system decors, including virtual displays enabled
  with FLAG_SHOULD_SHOW_SYSTEM_DECORATIONS, which is not allowed to
  dynamically add / remove system decors.
- This new API return true if the display is either default display, or
  the display is allowed to dynamically add / remove system decors, and it
  should currently show system decors.

The client of this API will be WMShell, such as DesktopModeStatus and
DesktopDisplayModeController. This API will be used to check whether a
display is eligible for the desktop mode.

Bug: 400808149
Test: adb shell settings put secure mirror_built_in_display {1|0}
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Change-Id: I596fae668403f6490d5c27f333324ee073ff185f