Fix session messages for affiliated HSUM DO

Under the hood setStartUserSessionMessage() sets the message to show
when _leaving_ main user, conversely setEndUserSessionMessage() sets
the message to show when _switching back_ to the main user.
On affiliated HSUM DO current code set it on USER_SYSTEM, while the
actual switch will happen from/to the main user. This CL uses main user
id in these cases.
This still will not work for mainless systems.

Bug: 433331731
Bug: 437845664
Test: manual, CTS-V
Flag: EXEMPT BUGFIX
Change-Id: I27daf5d7b054e258a48391968746874a32412987