build(android): sign release APKs with APK Signature Scheme v4

Release signing now also writes the V4 .apk.idsig beside each APK, verifies it in CI and publishes it for adb incremental installs. Normal installers keep using the V1-V3 signatures inside the APK.

android/app/build.gradle.kts

@@ -71,6 +71,9 @@ android {
                 enableV1Signing = true
                 enableV2Signing = true
                 enableV3Signing = true
+                // V4 lives in a separate .apk.idsig file used for
+                // `adb install --incremental`; the APK itself is unchanged.
+                enableV4Signing = true
             }
         }
     }