Make ViewPager example tabs visible over map content (#10608)

### Summary

Fixes [MAPSAND-2430](https://mapbox.atlassian.net/browse/MAPSAND-2430)

- `PagerTabStrip` in the ViewPager example had no background, rendering
tab text directly over map tiles — invisible on most map styles
- Added a semi-transparent dark background and white text color to
ensure readability

### Problem

`PagerTabStrip` rendered without a background, so its text was painted
directly on top of the map. On styles like globe, dark street, and
satellite, the text was effectively invisible.

### Solution

Added `android:background="#CC303030"` (semi-transparent dark overlay)
and `android:textColor="@android:color/white"` to the `PagerTabStrip` in
`activity_viewpager.xml`.

### Key Changes

- **File**:
`projects/maps-android/mapbox-maps-android/app/src/main/res/layout/activity_viewpager.xml`
- **Before** -> **After**: No background/text color -> `#CC303030`
background + white text

### Validation

- [x] Tested on Pixel 6 Pro (Android 16)
- [x] Tabs visible across globe, dark street, and satellite styles
- [x] No crashes

<img width="1440" height="3120" alt="viewpager-tabs-fix"
src="https://github.com/user-attachments/assets/f7a13672-ad31-4f51-af51-cdfee636bf6d"
/>

cc @mapbox/maps-android

GitOrigin-RevId: e20236125c03c31c3cba054c6819ab9c9866def7

Author: pengdev

app/src/main/res/layout/activity_viewpager.xml

@@ -13,6 +13,8 @@
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:layout_gravity="top"
+            android:background="@color/semi_transparent_dark"
+            android:textColor="@android:color/white"
             android:paddingBottom="4dp"
             android:paddingTop="4dp"/>
 

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

@@ -10,4 +10,5 @@
     <color name="red">#f74e4e</color>
     <color name="simi_transparent_blue">#334264fb</color>
     <color name="black">#000000</color>
+    <color name="semi_transparent_dark">#CC303030</color>
 </resources>