Fix "invalid buffer slot" hardware composer error

For virtual displays we weren't calling setClientTargetSlotCount(),
leading to "invalid buffer slot" errors from the hardware composer
service.

Bug: 69814692

Test: Observed the "invalid buffer slot" errors are gone.

Change-Id: I2268acd7d59dcbe2a943ddf388d88632a8b81b5d
(cherry picked from commit 8f1110776981bf35d5952532f99f32d9420dc017)