Align naming between the feature and testing config

Bug: 1124434
Change-Id: I3f4252c3b69809ad6966202e7788b2368dc1385d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391612Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811821}

testing/variations/fieldtrial_testing_config.json

@@ -4619,7 +4619,7 @@
             ]
         }
     ],
-    "OmniboxAssistantVoiceSearchAndroid": [
+    "OmniboxAssistantVoiceSearch": [
         {
             "platforms": [
                 "android"