android: stores website snapshots for thumb previews

https://discuss.privacyguides.net/t/most-of-the-android-browsers-store-full-size-website-snapshot-as-thumbnail-temporarily-root-directory-in-private-mode/19957

On Android, Firefox OG, nightly, Tor Browser (Image file extension is changed but still image can be open with file manager),Fennec etc. + most Chromium based browsers including BRAVE have this issue in their private browsing modes. Website snapshots are taken and stored on disk temporarily to be used as tab view thumbnails.

Firefox Focus and its Nightly don’t have this issue since they are single tab browsers. Firefox Focus Nightly + about:config, disk cache disabled seems to run all cache files in memory.

just FYI, also cc @dan @clairehurst