Cleanup several warnings inside DisplayMetricsHolder (facebook#51074)

Summary:
Pull Request resolved: facebook#51074

While working on this class I noticed there are a number of warnings. Let's clean them up.

Changelog:
[Internal] [Changed] -

Reviewed By: mdvacca

Differential Revision: D74001269

fbshipit-source-id: 7e86ffdaf9c82ff5a165618813fa700f8e850a74

Author: cortinico

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/DisplayMetricsHolder.kt

@@ -27,7 +27,7 @@ public object DisplayMetricsHolder {
   /** The metrics of the window associated to the Context used to initialize ReactNative */
   @JvmStatic
   public fun getWindowDisplayMetrics(): DisplayMetrics {
-    checkNotNull(windowDisplayMetrics) { DisplayMetricsHolder.INITIALIZATION_MISSING_MESSAGE }
+    checkNotNull(windowDisplayMetrics) { INITIALIZATION_MISSING_MESSAGE }
     return windowDisplayMetrics as DisplayMetrics
   }
 
@@ -39,7 +39,7 @@ public object DisplayMetricsHolder {
   /** Screen metrics returns the metrics of the default screen on the device. */
   @JvmStatic
   public fun getScreenDisplayMetrics(): DisplayMetrics {
-    checkNotNull(screenDisplayMetrics) { DisplayMetricsHolder.INITIALIZATION_MISSING_MESSAGE }
+    checkNotNull(screenDisplayMetrics) { INITIALIZATION_MISSING_MESSAGE }
     return screenDisplayMetrics as DisplayMetrics
   }
 
@@ -63,7 +63,6 @@ public object DisplayMetricsHolder {
     val screenDisplayMetrics = DisplayMetrics()
     screenDisplayMetrics.setTo(displayMetrics)
     val wm = context.getSystemService(Context.WINDOW_SERVICE) as WindowManager
-    checkNotNull(wm) { "WindowManager is null!" }
     // Get the real display metrics if we are using API level 17 or higher.
     // The real metrics include system decor elements (e.g. soft menu bar).
     //
@@ -75,8 +74,8 @@ public object DisplayMetricsHolder {
 
   @JvmStatic
   public fun getDisplayMetricsWritableMap(fontScale: Double): WritableMap {
-    checkNotNull(windowDisplayMetrics) { DisplayMetricsHolder.INITIALIZATION_MISSING_MESSAGE }
-    checkNotNull(screenDisplayMetrics) { DisplayMetricsHolder.INITIALIZATION_MISSING_MESSAGE }
+    checkNotNull(windowDisplayMetrics) { INITIALIZATION_MISSING_MESSAGE }
+    checkNotNull(screenDisplayMetrics) { INITIALIZATION_MISSING_MESSAGE }
 
     return WritableNativeMap().apply {
       putMap(