ci(android): fix split build, produce universal APK in the same Gradle run

The previous commit made `assembleRelease` depend on AGP's
`packageReleaseUniversalApk`. That task derives the universal APK from an
App Bundle, which pulls `buildReleasePreBundle` into the graph - and AGP
refuses to build a bundle while ABI splits are enabled
("Multiple shrunk-resources files found ... Please disable building
multiple APKs when building an Android app bundle", run 52).

Instead only flip `splits.abi.universalApk` back on when Flutter passes
`-Psplit-per-abi=true`: the Flutter Gradle plugin enables the splits and
copies every output - including the universal one, as app-release.apk -
to build/app/outputs/flutter-apk/. Plain builds (`flutter run`) keep the
plugin's abiFilters path, which AGP does not allow together with splits.

Workflow: one `flutter build apk --release --split-per-abi` call, copy the
four well-known output names (a missing one fails the step), job timeout
60 min (a green run takes ~17), drop the unneeded platform pre-install and
add a per-ref concurrency group - runs 50/51 ran in parallel on the single
ARM64 runner and both died at the same instant.

Verified locally with Flutter 3.47.4: split build yields all four APKs
(universal 62 MB, per-ABI 20-24 MB) in one Gradle run; plain release
build still produces app-release.apk.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

android/app/build.gradle

@@ -34,26 +34,29 @@ android {
     buildTypes {
         release {
             // TODO: Add your own signing config for the release build.
+            // Signing with the debug keys for now, so `flutter run --release` works.
             signingConfig signingConfigs.debug
         }
     }
 
-    splits {
-        abi {
-            enable true
-            reset()
-            include "arm64-v8a", "armeabi-v7a", "x86_64"
-            universalApk true
+    // `flutter build apk --split-per-abi` makes the Flutter Gradle plugin enable ABI
+    // splits for the requested target platforms but switch the universal APK off.
+    // Turn it back on so a single Gradle run yields the per-ABI APKs *and* the
+    // universal one (the plugin copies all of them to build/app/outputs/flutter-apk/,
+    // the universal one as app-release.apk). Only when splitting: for plain builds
+    // the plugin sets ndk.abiFilters instead, which AGP does not allow together
+    // with splits. Do not use AGP's `packageReleaseUniversalApk` task for this -
+    // it derives the universal APK from an App Bundle, and bundles cannot be
+    // built while ABI splits are enabled.
+    if (project.findProperty("split-per-abi")?.toString()?.toBoolean()) {
+        splits {
+            abi {
+                universalApk true
+            }
         }
     }
 }
 
-afterEvaluate {
-    tasks.named('assembleRelease').configure {
-        dependsOn 'packageReleaseUniversalApk'
-    }
-}
-
 kotlin {
     compilerOptions {
         jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_17