Fix bugreport warning dialog

This bug was introduced when fixing dark theme for bugreport dialogs.

Bug: 135446537
Test: manually (both in light and dark theme)
Change-Id: Ib8c16d068c00632d009033c2eab7d2d6cb7e4eea

packages/Shell/AndroidManifest.xml

@@ -235,6 +235,7 @@
 
         <activity
             android:name=".BugreportWarningActivity"
+            android:theme="@android:style/Theme.DeviceDefault.Dialog.Alert"
             android:finishOnCloseSystemDialogs="true"
             android:excludeFromRecents="true"
             android:exported="false" />