Start/Stop CPU monitoring based on the client callback addition/removal.

- Start CPU monitoring when the first client callback is added.
- Stop CPU monitoring when the last client callback is removed.
- When on client callbacks are available, perform debug monitoring only
  user-debug and eng builds.
- When a client callback is added, notify the client of the latest CPU
  availability and the current monitoring interval.

Test: atest CpuMonitorServiceTest
Bug: 242722241
Change-Id: I6d423ee165ad6e0623f6078e3c68cc4fd1871371