Export boot receiver explicitly

Android 12 requires explicit exports.
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index b97ffbe..b586e60 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -29,7 +29,8 @@
         android:supportsRtl="true">
 
         <receiver android:name=".BootReceiver"
-            android:directBootAware="true">
+            android:directBootAware="true"
+            android:exported="true">
             <intent-filter>
                 <action android:name="android.intent.action.LOCKED_BOOT_COMPLETED" />
             </intent-filter>