Make DownloadOverMeteredDialog not exported

The DownloadOverMeteredDialog is exported which may
allow a app to prompt this dialog to ask the user to
download dictionary.

Since the app can control the download dictionary size
and may show an incorrect size for user to make security
concern. Make the activity not exported for apps to launch.

Bug: 236688064
Test: Manual test as follow steps:
      1. lunch aosp_bramble-userdebug and flash
      2. Verify the AOSP keyboard work properly
      3. Launch the PoC test app
      4. Verify the DownloadOverMeteredDialog cannot be launched
Change-Id: I9be92ba2813e7aaad0ef10555f585c5c8a25361f