Bug 40002: Ensure system download manager is not used

Fixes #40002 (closed).

Comparing to fennec, I think these changes are equivalent to our changes in tor-browser#25741 (closed) (by setting MOZ_ANDROID_DOWNLOADS_INTEGRATION = false). I also put the MediaStore usage behind this flag, as it seems it's the new way of doing addCompletedDownload.

With these changes, running the command apkanalyzer dex packages [final.apk] | grep 'android.provider.MediaStore\|android.app.DownloadManager' should find no ocurrence. So, even though I think the default AndroidDownloadManager value in DownloadFeature.kt is not used in Fenix, removing it allows us to check android.app.DownloadManager is unused via this command.

This is something we could run somewhere to monitor whether this changes in future versions/rebases. We do similar things in tor-browser-bundle-testsuite, but perhaps checking this in the tor-browser-build fenix or tor-browser build script would be good enough for now (similarly to the idea in tor-browser-build#40093 (closed)).