SF: Extract processHotplug from configureLocked

As a precondition, assert that disconnected displays are known, i.e. a
display token exists.

Bug: 241285876
Test: libsurfaceflinger_unittest
Change-Id: I42e183390d29a8fe9b2cd4837a94843efb2e1d2c
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index bbe1589..2b230b3 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -915,6 +915,11 @@
     bool configureLocked() REQUIRES(mStateLock) REQUIRES(kMainThreadContext)
             EXCLUDES(mHotplugMutex);
 
+    // Returns a string describing the hotplug, or nullptr if it was rejected.
+    const char* processHotplug(PhysicalDisplayId, hal::HWDisplayId, bool connected,
+                               DisplayIdentificationInfo&&) REQUIRES(mStateLock)
+            REQUIRES(kMainThreadContext);
+
     sp<DisplayDevice> setupNewDisplayDeviceInternal(
             const wp<IBinder>& displayToken,
             std::shared_ptr<compositionengine::Display> compositionDisplay,