instrumentation tests: Stop passing a generator to posixpath.join()

> Commit d4d66d4 ("Reland #2: Move side-loaded test data /sdcard ->
> /sdcard/gtestdata") introduced a regression in
> LocalDeviceInstrumentationTestRun.substitute_device_root() when it's
> passed a list: the posixpath.join() call ends up receiving a generator
> instead of multiple arguments.
>
> This means the call will return a generator instead of 'foo/bar/baz',
> which is not what's intended.
>
> [email protected]
> Review-Url: https://codereview.chromium.org/2360693002

build/android/pylib/local/device/local_device_instrumentation_test_run.py

@@ -63,7 +63,7 @@ def substitute_device_root(d, device_root):
       if not d:
         return device_root
       elif isinstance(d, list):
-        return posixpath.join(p if p else device_root for p in d)
+        return posixpath.join(*(p if p else device_root for p in d))
       else:
         return d