SF: Fix pacesetter promotion for folded mirroring

The pacesetter display is demoted/promoted in response to a hotplug. On
foldables, the promoted display was hard-coded to the inner display, so
a hotplug while folded would incorrectly use that powered-off display as
the pacesetter, causing system-wide jank until the next fold/unfold.

Fixes: 347248313
Flag: EXEMPT bugfix
Test: Connect and disconnect external display while folded.
Test: Fold and unfold while external display is connected.
Test: Pacesetter is still correct on folded/unfolded boot.
Change-Id: Id5cb29c3cbaa8ed455a15d8be3a32e79a470cce5