Skip reading config file that does not exist

By default there is no such file. This avoids printing
exception log on every boot. Especially it prints out
thousands of callstack when running unit tests.

Bug: 159102753
Test: LetterboxConfigurationPersisterTest
Change-Id: I64a62c4aa66ac045aa4b1ddf1ad2e51d7bf4ca5b

services/core/java/com/android/server/wm/LetterboxConfigurationPersister.java

@@ -216,6 +216,10 @@ class LetterboxConfigurationPersister {
     }
 
     private void readCurrentConfiguration() {
+        if (!mConfigurationFile.exists()) {
+            useDefaultValue();
+            return;
+        }
         FileInputStream fis = null;
         try {
             fis = mConfigurationFile.openRead();