Update some comments for tuner AIDL HAL.

*) Clarify the meaning and unit of FrontendStatus.
*) Add comment for STC ready support.
*) Support for control signal (e.g. TMCC) change monitoring

Bug: 183634950
Bug: 202247846
Bug: 183630794
Test: Make
Change-Id: I3c48993c354e865b0edfe56adbd34c7fce120efc