Document permissions for WebViewUpdateManager.

WebViewUpdateManager.getValidWebViewPackages() requires
INTERACT_ACROSS_USERS and QUERY_ALL_PACKAGES - this is already enforced
by the service implementation but was not documented in the client API.

Bug: 319292658
Test: n/a
Flag: android.webkit.update_service_ipc_wrapper
Change-Id: I5a789b89235b328489ccb4f24f77f5f19500e059