Fix for heap-use-after-free in GPUService.cpp

This adds a unit test and fix for the bug reported by libfuzzer.
Changes made:
 * Expose GPUService as testable code.
 * Update main_gpuservice.cpp to use the new GpuService now located at
   gpuservice/GpuService.h
 * Make initializer threads members of GpuService
 * Join the threads in destructor to prevent heap-use-after-free.
 * Add unit test that waits 3 seconds after deallocation to ensure no
   wrong access is made.

Bug: 282919145
Test: Added unit test and ran on device with ASAN
(cherry picked from commit 3c00cbc0)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:7fb707802ee4c667d1ee6065ae2845d835b47aeb)
Merged-In: I4d1d2d4658b575bf2c8f425f91f68f03114ad029
Change-Id: I4d1d2d4658b575bf2c8f425f91f68f03114ad029