2018 Tor Browser Android donation banner

In legacy/trac#23925 (moved) we are implementing a donation banner for the YE 2018 campaign. It would be great to have a donation banner for Tor Browser Android as well. We already have the text in https://gitweb.torproject.org/torbutton.git/tree/src/chrome/locale/en/aboutTor.dtd#n30 getting translated.

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