Running git-annex linux builds in termux seems to work well enough that the
only reason to keep the Android app would be to support Android 4-5, which
the old Android app supported, and which I don't know if the termux method
works on (although I see no reason why it would not).
According to [1], Android 4-5 remains on around 29% of devices, down from
51% one year ago.

[1] https://www.statista.com/statistics/271774/share-of-android-platforms-on-mobile-devices-with-android-os/

This is a rather large commit, but mostly very straightfoward removal of
android ifdefs and patches and associated cruft.

Also, removed support for building with very old ghc < 8.0.1, and with
yesod < 1.4.3, and without concurrent-output, which were only being used
by the cross build.

Some documentation specific to the Android app (screenshots etc) needs
to be updated still.

This commit was sponsored by Brett Eisenberg on Patreon.