Modernize TimeCheck

This replaces the usage of Thread, Mutex, Condition and KeyedVector
with their std:: counterparts.

We're also extracting the time thread from TimeCheck and adding a unit
test.

The logic in TimerThread now is easier to follow and it is more
evident that tasks would never be executed before their deadline
arrived. In the original code, this wasn't obvious.

Test: atest libmediautils_test
Bug: 188748333
Change-Id: Iac887e7d6e6758c0a0e5c56695e9671082e64b09