Update bugreport dialog to DeviceDefault theme.

By using DeviceDefault instead of Material, this UI is now
resilient to any platform-level theme changes.

Change-Id: I43ce61b36f4c089ee07f754088abe2dfe6700877
Fixes: 30173174
diff --git a/packages/Shell/AndroidManifest.xml b/packages/Shell/AndroidManifest.xml
index 2efefb3..f1789ea 100644
--- a/packages/Shell/AndroidManifest.xml
+++ b/packages/Shell/AndroidManifest.xml
@@ -142,7 +142,7 @@
 
         <activity
             android:name=".BugreportWarningActivity"
-            android:theme="@android:style/Theme.Material.Light.Dialog.Alert"
+            android:theme="@android:style/Theme.DeviceDefault.Light.Dialog.Alert"
             android:finishOnCloseSystemDialogs="true"
             android:excludeFromRecents="true"
             android:exported="false" />