Reserve flags for CSS/WM features:

- multi_window_screen_context
- contextual_search_window_layer

These will guard the API entrypoints, which call into
dedicated codepaths in the WM for window-level operations.

Change-Id: I03ffe16dbcc4b51d32eb953f75813af3551bae11
Test: N/A flag declarations only
Flag: EXEMPT flag declarations only
Bug: 371065456
Bug: 372510690