Use user context for the clipboard overlay window

The clipboard overlay uses TYPE_SCREENSHOT, which doesn't show across
users. The window context needs to be constructed for the current user.

Bug: 217922018
Test: manual via adding a secondary user, verified that text copied
shows the clipboard overlay
Flag: com.android.systemui.clipboard_overlay_multiuser

Change-Id: I7d1c8edb209890189b81646f8702bd07691d5abd