Updated Android verions supported

Android API 15 no longer supported as onesignal.com requires TLS 1.2.
Android 12 has been tested so updated this.

Author: jkasten2

README.md

@@ -28,4 +28,4 @@ For account issues and support please contact OneSignal support from the [OneSig
 To make things easier, we have published demo projects in the `/Examples` folder of this repository.
 
 #### Supports:
-* Tested from Android 4.0.3 (API level 15) to Android 11 (30)
+* Tested from Android 4.1.0 (API level 16) to Android 12 (31)