Mark getStagedSessionErrorMessage as @NonNull.

Change-Id: I269175f24ef84e7e8b053528ab1074dad84bb6fe
Fix: 126698886
Test: n/a
diff --git a/api/current.txt b/api/current.txt
index efd2170..a22262a 100644
--- a/api/current.txt
+++ b/api/current.txt
@@ -11460,7 +11460,7 @@
     method public int getSessionId();
     method public long getSize();
     method public int getStagedSessionErrorCode();
-    method public String getStagedSessionErrorMessage();
+    method @NonNull public String getStagedSessionErrorMessage();
     method public android.os.UserHandle getUser();
     method public boolean isActive();
     method public boolean isMultiPackage();
diff --git a/core/java/android/content/pm/PackageInstaller.java b/core/java/android/content/pm/PackageInstaller.java
index b20cce9..7de8105 100644
--- a/core/java/android/content/pm/PackageInstaller.java
+++ b/core/java/android/content/pm/PackageInstaller.java
@@ -2130,7 +2130,7 @@
          * Text description of the error code returned by {@code getStagedSessionErrorCode}, or
          * empty string if no error was encountered.
          */
-        public String getStagedSessionErrorMessage() {
+        public @NonNull String getStagedSessionErrorMessage() {
             checkSessionIsStaged();
             return mStagedSessionErrorMessage;
         }