Reject a multi-sessin set as APEX (1/n)

Prevent the crash loop in b/163288861#comment1.

It doesn't make sense to set a multi-session to APEX at all.
We should check and reject this case.

Bug: 163288861
Test: N/A, will be added in next CL

Change-Id: I23147837a11733b776b1147797b545b539c6c05a