ci: restrict default GITHUB_TOKEN permissions to contents: read

None of these workflows write through GITHUB_TOKEN: artifact upload,
Firebase, S3, Play Store, gh-pages and the Appium dispatch all use their
own credentials. Reusable workflows they call inherit the same cap.

.github/workflows/code-quality.yml

@@ -29,6 +29,9 @@ env:
   DEBUG_GOOGLE_OAUTH_ID: ${{ secrets.DEBUG_GOOGLE_OAUTH_ID }}
   RELEASE_GOOGLE_OAUTH_ID: ${{ secrets.RELEASE_GOOGLE_OAUTH_ID }}
 
+permissions:
+  contents: read
+
 jobs:
   ktlint:
     name: Kotlin Lint