Create a mobly folder under tools/test/mobly_snippets as a Copybara
destination.

Our team and the Wifi team co-develop the snippet codes on Google3. To
share the snippet codes with dev team's multi-devices CTS test in
Android Gerrit, we need to build a Copybara channel to sync codes from
Google3 (Piper, as source of trust) to Android Gerrit (go/ag).

Bug: 239062528

Test: N/A
Change-Id: I1559ad77c925defcaffb25e3546d5417a9bc86c3