[android] slightly adjust zoom button spacing

results in the in-between spacing being the same height as the rounding, looks a tad more consistent

Signed-off-by: Harry Bond <me@hbond.xyz>

android/app/src/main/res/layout/map_buttons_zoom.xml

@@ -12,7 +12,7 @@
     android:tint="?iconTint"
     app:srcCompat="@drawable/ic_plus"
     app:shapeAppearanceOverlay="@style/ShapeAppearance.MapButton.Zoom.Minus"
-    android:layout_marginBottom="@dimen/margin_eighth"
+    android:layout_marginBottom="@dimen/margin_eighth_plus"
     android:contentDescription="@string/zoom_in"/>
   <com.google.android.material.floatingactionbutton.FloatingActionButton
     android:id="@+id/nav_zoom_out"

android/app/src/main/res/values/dimens.xml

@@ -8,6 +8,7 @@
 
   <!-- margins -->
   <dimen name="margin_eighth">2dp</dimen>
+  <dimen name="margin_eighth_plus">3dp</dimen>
   <dimen name="margin_quarter">4dp</dimen>
   <dimen name="margin_quarter_plus">6dp</dimen>
   <dimen name="margin_half">8dp</dimen>