Fix a crash in Storage Settings.

This prevents a NullPtrException when the DonutView attempts to shrink
the percentage sign. By gracefully falling back when we don't understand
the percentage sign, we can avoid a crash.

Change-Id: I0b0770ce4b51eb30817dc556636f86476f4cd1bf
Fixes: 77653937
Test: Robotest -- screenshot attached

tests/robotests/src/com/android/settings/widget/DonutViewTest.java

@@ -0,0 +1,19 @@
+package com.android.settings.widget;
+
+import android.content.Context;
+
+import com.android.settings.testutils.SettingsRobolectricTestRunner;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RuntimeEnvironment;
+
+@RunWith(SettingsRobolectricTestRunner.class)
+public class DonutViewTest {
+    @Test
+    public void getPercentageStringSpannable_doesntCrashForMissingPercentage() {
+        Context context = RuntimeEnvironment.application;
+
+        DonutView.getPercentageStringSpannable(context.getResources(), "50%", "h");
+    }
+}