Make a special remote using adb to send file to an android device.

While there is an android port, a special remote will suffice for many use
cases, and may work better overall.

It should support exporttree=yes, since most use cases involve exporting a
tree of files and consuming them on the android device.

Use adb push, adb pull, and use adb shell for checkpresent and remove.

Ought to implement [[import tree]] too, so that changes made
to files on the android device can be imported back into the git
repository.

And, [[export preferred content]] would be a useful feature for
excluding some files from a tree exported to android.

> Status: Remote implemented, but not yet [[import tree]] and
> [[export preferred content]]. --[[Joey]]
