release: set our release channel to beta now

support branch builds should not clobber alpha releases, alpha
releases go to alpha first every time, but the builds from this
branch should start on beta

AnkiDroid/build.gradle

@@ -242,7 +242,7 @@ android {
 
 play {
     serviceAccountCredentials.set(file("${homePath}/src/AnkiDroid-GCP-Publish-Credentials.json"))
-    track.set('alpha')
+    track.set('beta')
 }
 amazon {
     securityProfile = file("${homePath}/src/AnkiDroid-Amazon-Publish-Security-Profile.json")