Improve API of requestFullscreenMode

This patch improves the documentation and annotation of the API.
1. Added explanation of the type of Throwable: IllegalStateException
for all cases and will contain a detailed message to explain the
reason.
2. Explained that the activity need to be the focused display
top-most one.
3. Removed non-necessary @NonNull annotation
4. Documented the const of the request types.

Bug: 262491455
Test: atest CtsWindowManagerDeviceTestCases:FreeformWindowingModeTests
Change-Id: I950ef0aca9e7aa73e97cc3dd35f7f101ea59f863