SF: Cleanup EventThread Part 1

The cleanups in this CL are primarily about switching from
android::Thread to std::thread.

1) Convert from android::Thread to std::thread, along with using
std::mutex and std::condition_variable to keep consistency.
2) Switch the header to #pragma once.
3) Added Clang thread annotations and enabled the corresponding warning.
4) Added proper thread shutdown handling (invoked by dtor).

Test: No issues observed on Pixel XL
Bug: None

Change-Id: I2ef0ad18ef75e139f70d856031991f87d187efe6