SF: Fix many flakes in IdleTimerTest

There were a few flakes that were possible due to the way the idle
thread could end up being scheduled. In particular the following
tests have historically been flaky:

IdleTimerTest.idleTimerIdlesTest
IdleTimerTest.noCallbacksAfterStopAndResetTest
IdleTimerTest.noCallbacksAfterStopTest
IdleTimerTest.resetTest
IdleTimerTest.startStopTest
IdleTimerTest.timeoutCallbackExecutionTest

One thing that helps is to boost the priority of the test process as a
whole so that the IdleTimer thread is scheduled when it wants to be run.

But even then there are no guarantees that requesting a callback after
"3ms" actually generates that callback that quickly.

The adjustments to the test include:

1) Removing calls to sleep_for, in favor of doing follow-up operations
immediately after (start() then stop() for example), or relying on the
ability of the AsyncCallRecorder to wait for callbacks.
2) For startStopTest, using a larger interval, and sanity checking with
a clock source that an unexpected event really is unexpected.
3) For resetTest also using a long interval to observe the behavior if
the reset happens shortly before a callback for the previous interval
would be made.

There was unfortunately one change necessary to the implementation, not
just the test. It turned out that the condition.wait_for could return
after the interval had expired without returning
std::cv_status::timeout, and therefore not trigger a callback even
though it should have. This lead to idleTimerIdlesTest being flakey even
with the other changes.

There was also second issue discovered in Scheduler, where it did not
shut down the thread properly in its destructor, which could allow the
callback the Scheduler sets to be invoked while the Scheduler instance
is being destroyed (leading to lock attempt on a destroyed mutex).

Test: libsurfaceflinger_unittest --gtest_repeat=1000  # ALL pass x1000
Test: atest libsurfaceflinger_unittest # All, not just IdleTimerTest
Bug: 122319747
Change-Id: I716451524c32cc6a299523c47c11cfefd6ab4460