PiP: Start PiP settings as current user.

Previously since PipMenuActivity was started as current user, starting
the new settings activity as the same user works. However now that
Settings activity starts from SysUI directly (which would be SysUI's
primary user), that no longer works - we need to now be explicit about
using current user again.

Bug: 166468109
Test: Enter PIP, press settings gear icon - Settings open
Change-Id: Iaf257405a38d026ff576e747a6a443ab9e4dd8b5