Generate random challenge in RKP factory tool

The challenge was previously hard-coded to an empty string previously.
Though it's not necessarily required for remote key provisioning
certificate signing requests, go ahead and include good randomness
(via getrandom) into the csr.

Ignore-AOSP-First: No merge path to aosp, will manually merge
Bug: 191301285
Test: Manually run rkp_factory_extraction_tool
Change-Id: I68e7d86259c67d40ecbb0c9e4ecac22954757dd3
1 file changed