Use CATEGORY_SECONDARY_HOME on CD during drag-to-desktop transition

Otherwise the default launcher activity is used instead of
SecondaryDisplayLauncher, which causes the start transition to not
contain the transient home change nor the dragging task's to-back change
leading to an illegal state exception.

Flag: EXEMPT BUGFIX
Fix: 444506626
Test: with a tablet, launch desktop app in primary display, move to
fullscreen, then drag handle to enter desktop, then close it. Repeat in
the external display - verify it can enter desktop with no crash

Change-Id: I3161633a45579b71a78a39128247a7bf538aa1ec