ContextHubService: Simplify hubId logic, ensure type correctness

Update nativeSendMessage() to simplify the logic for mapping the hub
handle to hub ID, and also update the types of some variables to ensure
their correctness. Also, rewrite unnecessary alias declarations as
simpler using declarations.

Test: run hello world context hub GTS test
Change-Id: I8f9154a0ff2c81eff9eb509e9b0936e34569ec55