Fixed gpuservice_unittest mem leaks

  Updated GpuStatsTest to destroy GpuStats instance in advance.
  It enables binder to process pending transactions prior test is
  destroyed, preventing memory leaks.

Bug: 222527290
Test: atest gpuservice_unittest
Change-Id: Icf9dfe895b835f2be2d0f69d74e978277b9e1579