Resolve Bug 40075: Support scoped storage to enable downloads on API >= 29

linked issues

• closes #40075 (closed)
• closes fenix#40192 (closed)

context

• in !7 (merged), we blocked all usage of Scoped Storage (via an BuildConfig.ANDROID_DOWNLOADS_INTEGRATIONtoggle) in an attempt to block usage of the system AndroidDownloadManager, which is known to cause proxy bypasses
• as of Android API 29, downloads will not work without Scoped Storage, causing all downloads to fail (see: fenix#40192 (closed))

changes

• enable usage of scoped storage for API >= 29
• abandon attempts to block writing metadata about downloaded files to system databases (as justified here), which frees us to...
• eliminate the ANDROID_DOWNLOADS_INTEGRATION feature toggle altogether