Aperture: Replace hardcoded targetSdkVersion with defaultConfig variable

Change-Id: Iab9e5ace963bfae1f969ef20e39e7df027c7b022
diff --git a/app/build.gradle.kts b/app/build.gradle.kts
index 9e8a463..4927306 100644
--- a/app/build.gradle.kts
+++ b/app/build.gradle.kts
@@ -211,7 +211,7 @@
                     ret
                 }
 
-        var targetSdkVersion = 31
+        var targetSdkVersion = android.defaultConfig.minSdk
         var minSdkVersion = 14
 
         // Extract AndroidManifest.xml for AARs