[avalanche] Add a flag to indicate if the node started voting in getavalanchinfo

Summary:
This makes it easier to figure out if the node is active on the network than computing from the staked amount and parameters.

Ref T1864.

Test Plan:

./test/functional/test_runner.py abc_rpc_getavalancheinfo

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Subscribers: tyler-smith

Maniphest Tasks: T1864

Differential Revision: https://reviews.bitcoinabc.org/D11283