chore(android): bump AGP to 8.9.1 and Java compatibility to 17

Transitive AndroidX dependencies (browser:1.9.0, core-ktx:1.17.0,
core:1.17.0) pulled in by the pub upgrade now require AGP 8.9.1+.

- AGP: 8.7.3 → 8.9.1
- Java source/target compatibility: 11 → 17
- Gradle 8.12 already satisfies AGP 8.9.1's minimum of 8.11.1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

android/settings.gradle.kts

@@ -18,7 +18,7 @@ pluginManagement {
 
 plugins {
     id("dev.flutter.flutter-plugin-loader") version "1.0.0"
-    id("com.android.application") version "8.7.3" apply false
+    id("com.android.application") version "8.9.1" apply false
     id("org.jetbrains.kotlin.android") version "2.1.0" apply false
 }