Add explicit SDK version to messaging AAR. (#1523)

* Add SDK version to messaging AAR.

* Add readme.

* Switch minSdkVersion to 14.

Author: jonsimantov

messaging/messaging_java/build.gradle

@@ -43,6 +43,10 @@ android {
   }
   compileSdkVersion 33
 
+  defaultConfig {
+    minSdkVersion 14
+  }
+
   sourceSets {
     main {
       manifest.srcFile '../src/android/java/LibraryManifest.xml'

release_build_files/readme.md

@@ -636,6 +636,8 @@ code.
     - Dynamic Links: The Dynamic Links SDK is now deprecated. See the [support
       documentation](https://firebase.google.com/support/dynamic-links-faq)
       for more information.
+    - Messaging (Android): Fixed minSdkVersion in the firebase_messaging.aar
+      manifest file.
 
 ### 11.7.0
 -   Changes