[fenix] Closes https://github.com/mozilla-mobile/fenix/issues/14009: Fix onboarding 'Sign in to Firefox' ripple effect (https://github.com/mozilla-mobile/fenix/pull/17169)

* Closes https://github.com/mozilla-mobile/fenix/issues/14009: Fix onboarding 'Sign in to Firefox' ripple effect

* For https://github.com/mozilla-mobile/fenix/issues/14009: Add 'onboarding_padded_background_color' instead of rely on OS theme/color

Both previous solution, '?android:attr/colorControlHighlight' and '@color/ripple_material_light' were using a OS theme attr or color, which caused problems. To fix it, we introduced a 'onboarding_padded_background_color' which manually define the ripple effect color of the onboarding manual sign in button.

app/src/main/res/drawable/onboarding_padded_background.xml

@@ -2,7 +2,7 @@
    - License, v. 2.0. If a copy of the MPL was not distributed with this
    - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
 <ripple xmlns:android="http://schemas.android.com/apk/res/android"
-    android:color="?android:attr/colorControlHighlight">
+    android:color="@color/onboarding_padded_background_color">
     <item
         android:bottom="6dp"
         android:top="6dp">

app/src/main/res/values/colors.xml

@@ -372,6 +372,7 @@
     <color name="mozac_feature_readerview_selected">@color/accent_high_contrast_dark_theme</color>
 
     <!-- Onboarding colors -->
+    <color name="onboarding_padded_background_color">#1F000000</color>
     <color name="onboarding_card_background_dark">#20123A</color>
     <color name="onboarding_card_primary_text_dark">#FFFFFF</color>
     <color name="onboarding_card_button_background_dark">#F9F9FB</color>