Prevent race condition by foregroundAppPackageName

Create a local variable from the member variable so it cannot become
null part way through.

Bug: 235064663
Test: Builds
Change-Id: Id5d13ee105fc2c78327ff3ddb3839f6cbd99fbab
diff --git a/services/core/java/com/android/server/display/AutomaticBrightnessController.java b/services/core/java/com/android/server/display/AutomaticBrightnessController.java
index 72464be..db646df 100644
--- a/services/core/java/com/android/server/display/AutomaticBrightnessController.java
+++ b/services/core/java/com/android/server/display/AutomaticBrightnessController.java
@@ -993,8 +993,9 @@
                     final String packageName = info.topActivity.getPackageName();
                     // If the app didn't change, there's nothing to do. Otherwise, we have to
                     // update the category and re-apply the brightness correction.
-                    if (mForegroundAppPackageName != null
-                            && mForegroundAppPackageName.equals(packageName)) {
+                    String currentForegroundAppPackageName = mForegroundAppPackageName;
+                    if (currentForegroundAppPackageName != null
+                            && currentForegroundAppPackageName.equals(packageName)) {
                         return;
                     }
                     mPendingForegroundAppPackageName = packageName;