Create platformtestprotos.

This target contains some proto definitions used by TradeFed. TF will
use this instead of platformprotos to reduce proto dependencies on the
platform.

A separate build file is created so its ownership is managed separately.
The OWNERS file will be updated due to merge conflicts.

Bug: 181921006
Test: New target builds.
Change-Id: I87e7a73d1160e3cf56a4c013e74989cdc90e1211
Merged-In: I82386a6c8a5e6ddf42d8377148001f4ede2f297f