Add Kotlin compiler session file.

android/app/build.gradle.kts

@@ -77,7 +77,9 @@ android {
 
     buildTypes {
         release {
-            signingConfig = signingConfigs.getByName("debug")
+            // signingConfig is set per flavor in productFlavors above.
+            // Setting null here lets the flavor-level config take effect.
+            signingConfig = null
         }
     }
 }