ci: build release APK and publish a GitHub Release on main

Adds a Build APK workflow: on every push to main it builds the release
APK with Flutter 3.35.3 and publishes it as a GitHub Release tagged
with the pubspec version (v1.7.0 → xtremobile-v1.7.0.apk). On pull
requests and manual runs the APK is built and uploaded as an artifact
only. Bumps app version to 1.7.0+14 for this release.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0156exq7fejVrD3HKU3MmS8X

pubspec.yaml

@@ -1,7 +1,7 @@
 name: xtremobile
 description: High-performance IPTV Android Application with Xtream Codes API
 publish_to: 'none'
-version: 1.6.1+13
+version: 1.7.0+14
 
 environment:
   sdk: '>=3.0.0 <4.0.0'