init: Avoid extra string copies when finding devices by using const refs

Now that FindPlatformDevice() isn't modifying the path argument, and
is just passing it on to FindSubsystemDevice(), take this argument by
const reference. That should avoid an unnecessary string copy.

Bug: 316324155
Test: Compile
Change-Id: I1d92a322d0c311ee46a117dd9d650896ec02520f