chore(ci): Stop backend system tests from running on Android-only PRs (JAVA-519)

Android-only PRs were silently triggering 24 unrelated backend system test jobs (~8 min each) on every push and PR. This commit adds path-ignore filters to exclude ~10 Android modules, so that backend CI tests will be skipped in the case of Android-specific changes.

Author: 0xadam-brown

.github/workflows/system-tests-backend.yml

@@ -4,7 +4,15 @@ on:
   push:
     branches:
       - main
+    paths-ignore:
+      - '*android*/**'
+      - 'sentry-compose/**'
+      - 'sentry-samples/sentry-samples-android/**'
   pull_request:
+    paths-ignore:
+      - '*android*/**'
+      - 'sentry-compose/**'
+      - 'sentry-samples/sentry-samples-android/**'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}