Bugfix: default trout_HOST_PROGRAM_PATH

The cmake rules look for prebuilt protoc and grpc_cpp_plugin if we build
without Android developing environment, but the default search path is
wrong.

Test: build without lunch, but use floral Clang instead
Bug: 155491613
Change-Id: I51acce7456ac682313c25c510bf5f0f9eca21823