Implement and integrate the verification token handler.

This CL implements a background task handler which for now, runs
a thread to retrieve verification tokens. This CL also integrates
this handler with the enforcement module, in order to allow operations
to receive verification tokens.

Bug: 171503362, 171503128
Test: TBD
Change-Id: I2ed0742043095dafb3b5cb7581ca3a2a70929ecc
6 files changed