[Android] Explicitly specified button type for rewards panel

Otherwise it falls back to MaterialButton and causes styling issues.

android/java/res/layout/brave_rewards_panel.xml

@@ -207,7 +207,7 @@
                             android:orientation="vertical"
                             android:visibility="gone"/>
 
-                        <Button
+                        <android.widget.Button
                             android:id="@+id/send_a_tip"
                             android:layout_height="50dp"
                             android:layout_width="fill_parent"
@@ -227,7 +227,7 @@
 
                     </GridLayout>
 
-                    <Button
+                    <android.widget.Button
                         android:id="@+id/rewards_summary"
                         android:layout_height="50dp"
                         android:layout_width="fill_parent"

android/java/res/layout/brave_rewards_panel_header.xml

@@ -40,7 +40,7 @@
                 android:layout_columnSpan="2"
                 android:visibility="visible"/>
 
-            <Button
+            <android.widget.Button
                     android:id="@+id/btn_verify_wallet"
                     android:layout_height="25dp"
                     android:layout_width="wrap_content"
@@ -165,7 +165,7 @@
                     android:gravity="center"
                     android:layout_gravity="center">
 
-                    <Button
+                    <android.widget.Button
                         android:id="@+id/br_add_funds"
                         android:layout_height="wrap_content"
                         android:layout_width="wrap_content"
@@ -184,7 +184,7 @@
                         android:visibility="gone"
                         tools:ignore="ButtonStyle"/>
 
-                    <Button
+                    <android.widget.Button
                         android:id="@+id/br_rewards_settings"
                         android:layout_height="wrap_content"
                         android:layout_width="wrap_content"