Add template listener for getting OEM netd

It is a template OemNetdListener that OEM could extend their
binder interface based on it.

Bug: 120251561
Test: built, flashed, booted
        system/netd/tests/runtests.sh pass

Merged-In: I2933eb09e5833a15507eecd898fb8d194098baa9
Merged-In: Ib1a0fd7c0868bb1a339c79d431f62f1385f36592
(cherry picked from commit 4315131c)

Change-Id: I2eb208724c2fb69dd0466c161ca2630524fd31c8