[Unity][Android] Clean up SyncAndServicesPreferences

This CL performs several minor cleanups in SyncAndServicesPreferences
without any changes to the behavior:
1. Reorder constants, fields and calls to have a consistent order for
   preferences.
2. Consistently use PREF_ prefix for preference keys.
3. Use mSync prefix for preferences that control sync data types.
4. Rename mSyncOmnibox to mSyncHistory to match the title string.
5. Replace onPreferenceClick and if chain with separate listener
   objects.

Bug: 814728
Change-Id: I60fda93757fa160e7c70b98925486332880c92d5
Reviewed-on: https://chromium-review.googlesource.com/1185200Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585513}

chrome/android/java/res/xml/sync_and_services_preferences.xml

@@ -32,11 +32,11 @@
             android:title="@string/sync_bookmarks"/>
         <org.chromium.chrome.browser.preferences.ChromeBaseCheckBoxPreference
             android:persistent="false"
-            android:key="payments_integration"
-            android:title="@string/payments_integration"/>
+            android:key="sync_payments_integration"
+            android:title="@string/sync_payments_integration"/>
         <org.chromium.chrome.browser.preferences.ChromeBaseCheckBoxPreference
             android:persistent="false"
-            android:key="sync_omnibox"
+            android:key="sync_history"
             android:title="@string/sync_history"/>
         <org.chromium.chrome.browser.preferences.ChromeBaseCheckBoxPreference
             android:persistent="false"

chrome/android/java/strings/android_chrome_strings.grd

@@ -1592,7 +1592,7 @@ To obtain new licenses, connect to the internet and play your downloaded content
       <message name="IDS_PAYMENTS_INTEGRATION_LEGACY" desc="Title for preference which enables import of Google Pay data for Autofill. 'Google Pay' should not be translated as it is the product name.">
         Cards and addresses using Google Pay
       </message>
-      <message name="IDS_PAYMENTS_INTEGRATION" desc="Title for preference which enables import of Google Pay data for Autofill. 'Google Pay' should not be translated as it is the product name.">
+      <message name="IDS_SYNC_PAYMENTS_INTEGRATION" desc="Title for preference which enables import of Google Pay data for Autofill. 'Google Pay' should not be translated as it is the product name.">
         Credit cards and addresses using Google Pay
       </message>
       <message name="IDS_SYNC_ACTIVITY_AND_INTERACTIONS_TITLE" desc="Title for preference which enables sync'ing of activity and interactions.">