Add Display#FLAG_ALLOWS_CONTENT_MODE_SWITCH

- For local display device and wifi display device, the flag should be
  enabled by default.

- For overlay display device used in CTS, the flag should be disabled
  by default; for other overlay display devices, the flag should be
  enabled by default to simulate connected displays.

- Virtual display device should not set this flag at this point.

Bug: 391965805
Test: atest DisplayContentTest
Test: modified tests in the same topic
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Change-Id: I7e6a31f8d5271e36de437854bf0c22d8275b1354