android.py: tracking_branch should be optional

|tracking_branch| is not required so we should not
call validate.require on it.

BUG=b:175901991
TEST=CQ

Change-Id: Ia8580bc5e6087936118421be4217d361d538ebda
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2596699
Commit-Queue: Boleyn Su <boleynsu@chromium.org>
Tested-by: Boleyn Su <boleynsu@chromium.org>
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
diff --git a/api/controller/android.py b/api/controller/android.py
index cb2e2a3..b3fe214 100644
--- a/api/controller/android.py
+++ b/api/controller/android.py
@@ -35,7 +35,7 @@
 
 @faux.success(_MarkStableResponse)
 @faux.empty_error
-@validate.require('tracking_branch', 'package_name', 'android_build_branch')
+@validate.require('package_name', 'android_build_branch')
 @validate.validation_complete
 def MarkStable(input_proto, output_proto, _config):
   """Uprev Android, if able.