Add display category for built in.

Add a category to get built in displays. A built in display is any
display that has the internal type.
Add a flag to guard the category.

Flag: com.android.server.display.feature.flags.display_category_built_in
Bug: 293651324
Test: atest CtsDisplayTestCases:DisplayManagerTest
Change-Id: I2d4f4de3bc34c11a8bf1b4649a0f354a96588c85