Log as warning when persistent setting file doesn't exist

The Thread persistent setting file may be missing if a new device is
started or the file is deleted on the device. Updated the logging from
error to warning as the file will be created.

Test: presubmit
Change-Id: I107712fc6fef61f26bc9be49fd703e61beae32a6
diff --git a/thread/service/java/com/android/server/thread/ThreadPersistentSettings.java b/thread/service/java/com/android/server/thread/ThreadPersistentSettings.java
index aba4193..9f34b8a 100644
--- a/thread/service/java/com/android/server/thread/ThreadPersistentSettings.java
+++ b/thread/service/java/com/android/server/thread/ThreadPersistentSettings.java
@@ -210,7 +210,7 @@
                 mSettings.putAll(bundleRead);
             }
         } catch (FileNotFoundException e) {
-            Log.e(TAG, "No store file to read", e);
+            Log.w(TAG, "No store file to read", e);
         } catch (IOException e) {
             Log.e(TAG, "Read from store file failed", e);
         }