Fix Wi-Fi Easy Connect buttons UI defects

1. Apply attr/colorAccent to button icons for theme UI control
2. Apply attr/selectableItemBackground for button tapping ripple effect
3. Use ConstraintLayout to separate ssid EditText and scan button
4. Remove ButtonPreference and add AddNetworkPreference.
   ButtonPreference's naming and design look like a general purpose UI
   component but it's not. This change refactors the code.
5. In AddNetworkPreference, use settingslib layout file
   'preference_access_point' to fix UI alignment problems.

Bug: 126964552
Bug: 125434239
Bug: 126762937

Test: manual test
      atest com.android.settings.wifi.WifiSettingsTest
Change-Id: Ib899a1e10f96bb8427ff00d6b5dfca37a0642c44

tests/robotests/src/com/android/settings/wifi/WifiSettingsTest.java

@@ -69,6 +69,7 @@ public class WifiSettingsTest {
         mWifiSettings = spy(new WifiSettings());
         doReturn(mContext).when(mWifiSettings).getContext();
         doReturn(mPowerManager).when(mContext).getSystemService(PowerManager.class);
+        mWifiSettings.mAddWifiNetworkPreference = new AddWifiNetworkPreference(mContext);
         mWifiSettings.mSavedNetworksPreference = new Preference(mContext);
         mWifiSettings.mConfigureWifiSettingsPreference = new Preference(mContext);
         mWifiSettings.mWifiTracker = mWifiTracker;
@@ -151,4 +152,11 @@ public class WifiSettingsTest {
         assertThat(mWifiSettings.mConfigureWifiSettingsPreference.getSummary()).isEqualTo(
                 mContext.getString(R.string.wifi_configure_settings_preference_summary_wakeup_off));
     }
-}
+
+    @Test
+    public void checkAddWifiNetworkPrefernce_preferenceVisible() {
+        assertThat(mWifiSettings.mAddWifiNetworkPreference.isVisible()).isTrue();
+        assertThat(mWifiSettings.mAddWifiNetworkPreference.getTitle()).isEqualTo(
+                mContext.getString(R.string.wifi_add_network));
+    }
+}