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