Ignore dev option force_desktop_mode_on_external_displays

when aflag enable_display_content_mode_management is enabled.

When dev option enable_display_content_mode_management is enabled,
DisplayContent#isSystemDecorationsSupported() alwasys return true. This
breaks the extended -> mirroring switch. The HOME activity remains on
the connected display even when the connected display is set to
mirroring mode.

Bug: 400344673
Test: manaually
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Change-Id: I6c2304e1cc8488aac969ad788e84d5c90af0da19