To run the standard library tests on Android, the androidtest.bash
script copies GOROOT to the device. Move that logic to the android
exec wrapper, thereby making androidtest.bash obsolete.

Apart from making Android less special, the sharded builder
infrastructure should now be able to run (emulated) Android builders
and trybots without special treatment.

Updates #23824

Change-Id: I41591fea9a15b38c6dcf84046ea57f1e9165eaa5
Reviewed-on: https://go-review.googlesource.com/c/163619
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>