Add logging when attempting to dream from power press.

Added a log statement to the logic that attempts to dream from a short
power press in order to identify black screen situations caused by an
inability to start dreaming (likely because the system thinks it is
already dreaming).

Bug: 282191629
Test: manually by pushing the power button when the device is already
dreaming and seeing that the log statement is emitted.

Change-Id: I2b18715677ba14c82694adf7480cbed1e8862231

services/core/java/com/android/server/policy/PhoneWindowManager.java

@@ -1086,6 +1086,8 @@ public class PhoneWindowManager implements WindowManagerPolicy {
 
         final DreamManagerInternal dreamManagerInternal = getDreamManagerInternal();
         if (dreamManagerInternal == null || !dreamManagerInternal.canStartDreaming(isScreenOn)) {
+            Slog.d(TAG, "Can't start dreaming when attempting to dream from short power"
+                    + " press (isScreenOn=" + isScreenOn + ")");
             noDreamAction.run();
             return;
         }