Create a new visual design for action buttons

- Create a new style for two action buttons.
- Change the layout of two_action_buttons.
  Since we use same style for positive/nagative
  buttons now, we only remain button1/button2 for
  this layout.
- Create a setButtonIcon interface in ActionButtonPreference.
  So, user can set icon for each button in this preferece.

Test: visual, robotest
Bug: 116346008
Change-Id: I511272cba5fd961349b85cae6d30004ddabe2c8e

tests/robotests/src/com/android/settings/bluetooth/BluetoothDetailsButtonsControllerTest.java

@@ -56,8 +56,8 @@ public class BluetoothDetailsButtonsControllerTest extends BluetoothDetailsContr
         super.setUp();
         final View buttons = View.inflate(
                 RuntimeEnvironment.application, R.layout.two_action_buttons, null /* parent */);
-        mConnectButton = buttons.findViewById(R.id.button2_positive);
-        mForgetButton = buttons.findViewById(R.id.button1_positive);
+        mConnectButton = buttons.findViewById(R.id.button2);
+        mForgetButton = buttons.findViewById(R.id.button1);
         mController =
             new BluetoothDetailsButtonsController(mContext, mFragment, mCachedDevice, mLifecycle);
         mButtonsPref = ActionButtonPreferenceTest.createMock();