Use subscription manager in VHAL.

This CL renames GetSetValuesClient to ConnectedClient and add
SubscribeClient to the supported client type. The SubscribeClient
would provide a different timeout and on-results callback.

This CL adds a map for subscribe clients in VHAL and initializes
subscription manager.

Test: None, will add after we implement subscribe.
Bug: 200737967
Change-Id: I222b155e4365dcf0fa9344c7da5901142433f055