Reduce flaky of DataUsageSummaryPreference

- Set SnapshotTimeMs initial value to unknown
- Reserve height for DataUsageSummaryPreference

Bug: 341234382
Test: manual - on Mobile Settings
Change-Id: I9145f666c5f8fd9595bcc047160fd6da17b3091d

res/layout/data_usage_summary_preference.xml

@@ -18,6 +18,7 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
+    android:minHeight="172dp"
     android:paddingTop="8dp"
     android:paddingBottom="16dp"
     android:paddingStart="?android:attr/listPreferredItemPaddingStart"
@@ -37,7 +38,6 @@
         android:id="@+id/usage_layout"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:paddingTop="12dp"
         android:orientation="horizontal">
 
         <TextView android:id="@+id/data_usage_view"