Add EXTRA_EXCLUDED_USERS key to DocumentsContract

This commit introduces `DocumentsContract.EXTRA_EXCLUDED_USERS`, a new
SystemApi extra. This allows callers of DocumentsUI intents (like
ACTION_OPEN_DOCUMENT) to provide an array of user IDs. These specified
users will then be excluded from the list of roots (e.g., user profiles)
displayed in the DocumentsUI picker.

Bug: 400701044
Bug: 360066001
Test: Verified using DocumentsUI picker
Test: atest CtsProviderUiTestCases
Flag: android.multiuser.enable_moving_content_into_private_space

Change-Id: I4f2d42c6d075902a8c53f58f0260ffb7a1e19467