SF: Fix UAF due to outliving idle timer

On Scheduler destruction, stop the timer and clear the callbacks, since
the RefreshRateConfigs may outlive the Scheduler.

On RefreshRateConfigs destruction, ensure the timer is destroyed before
the mutex/callbacks. Invoke the callback under lock.

Remove TestableSurfaceFlinger's subtle precondition that setupScheduler
must be called prior to FakeDisplayDeviceInjector for them to share
RefreshRateConfigs, which was not the case in DisplayModeSwitchingTest.

TestableScheduler is deleted through a Scheduler pointer, so ~Scheduler
should be virtual. Although ~TestableScheduler is trivial, this invoked
undefined behavior.

Bug: 213688734
Test: libsurfaceflinger_unittest --gtest_repeat=1000
Change-Id: Id23fafaf3d7071a5e28e275de386dd731a726006