Update some comments for tuner.

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

Bug: 183634950
Bug: 202247846
Bug: 183630794
Bug: 210845647
Test: Make
Change-Id: I6bc35ba723544692c15c0a293eef60215fa74f22