make python gRPC task parse import public in protos

[alexander-fenster]

import public in protos is a thing, and it seems like in Python there is a code that changes

import "google/cloud/securitycenter/v1/run_asset_discovery_response.proto";

to

import "google/cloud/securitycenter_v1/proto/run_asset_discovery_response.proto";

but this logic fails to update import public statement, which causes File not found error. This is a blind attempt to fix it (I don't see an easy way to verify that this fix works).

[yihanzhen]

Did you (is it possible) to verify it locally?

[alexander-fenster]

@hzyi-google I wasn't able yet :) What I will do is I will just run the whole smoke test here, I bet it will be easier than making it run locally.

[alexander-fenster]

@hzyi-google Running tests here, we'll see if securitycenter passes or not.

[alexander-fenster]

@hzyi-google It worked!

(this one failure is expected, it's the same as in master and unrelated; securitycenter worked though!)