Display: make Colors settings entry preference reactive

Added `display_color_mode` listener to Colors preference. As a result,
it becomes reactive and updates its color mode value summary.

Flag: EXEMPT minor change
Bug: 397659800
Test: changed color mode using `adb` commands and verify that Colors
summary reacts and print correct color mode

Change-Id: I963768e3dbb43b547ec53e6445b2791ec0f57cff