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.
diff --git a/api/controller/android_unittest.py b/api/controller/android_unittest.py
index 7fdca0e..2be0ce9 100644
--- a/api/controller/android_unittest.py
+++ b/api/controller/android_unittest.py
@@ -57,13 +57,6 @@
                      'android-package-name')
     self.assertEqual(self.response.android_atom.version, '1.2')
 
-  def testFailsIfTrackingBranchMissing(self):
-    """Fails if tracking_branch is missing."""
-    self.input_proto.tracking_branch = ''
-    with self.assertRaises(cros_build_lib.DieSystemExit):
-      android.MarkStable(self.input_proto, self.response, self.api_config)
-    self.uprev.assert_not_called()
-
   def testFailsIfPackageNameMissing(self):
     """Fails if package_name is missing."""
     self.input_proto.package_name = ''