Mark startRenderingCommandsCapture @UnsupportedAppUsage

This is to opt-out of hidden api enforcement essentially as
Studio needs to be able to access this symbol regardless
of if it's in an instrumentation test or not.

The API itself isn't really stable, nor a system API,
so making it public & @SystemApi respectively was discouraged.

So keeping it a @TestApi, indicating it's clearly coupled
to the current platform version, and using @UnsupportedAppUsage
to opt-out of hidden api enforcement seems to be the least-worst
combination.

Test: none
Change-Id: Id297107595cc7dfce62661940d5d61ee5efe4d7b