The android emulator builders is running out of space after CL 165797
copied most of GOROOT to the device.
The pkg directory is by far the largest, so only include what seems
necessary to build the x/ repositories: pkg/android_$GOARCH and
pkg/tool/android_$GOARCH.

While here, rename the device root directory to match the exec
wrapper name and make sure the deferred cleanups actually run before
os.Exit.

Hopefully fixes the emulator builders.

Updates #23824

Change-Id: I4d1e3ab2c89fd1e5818503d323ddb87f073094da
Reviewed-on: https://go-review.googlesource.com/c/go/+/166397
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>