Updates the dim for USER_SYSTEM if mCurrentUserId = null

If setWallpaperDimAmount is called during boot before
WallpaperManagerService#switchUser is called, it will produce a NPE
since the mCurrentUserId is still USER_NULL. Instead, update the dim for
USER_SYSTEM in that scenario.

Flag: NONE, the logic only changes in a case that would produce a NPE
otherwise.
Test: presubmit
Bug: 344545351

Change-Id: I3c8de053837e80fa6dc90f26b61e1f0f3af0eecf