Pass in enter reasons into SplitscreenEventLogger

* Multi-instance reason is kind of a default reason
if the enterSessionId hasn't been set.
* Explicitly set Launcher enter reasons so we don't
rely on Launcher being a default enter reason

Bug: 231369744
Test: Enter split screen from launcher, chrome new
window multi instance, and notification drag.
Logged enter reason when logging to verify.

Change-Id: I7af3b2c2b8c067ca4e6eadff037582d340cad0f5