Shutdown newFixedThreadPool and newSingleThreadExecutor.

The javadoc of newFixedThreadPool says:
> The threads in the pool will exist until it is explicitly shutdown.

The javadoc of newSingleThreadExecutor says:
> Unlike the otherwise equivalent newFixedThreadPool(1) the returned
> executor is guaranteed not to be reconfigurable to use additional
> threads.

So we have to manually shut them down, or we will leak them.

Bug: 259472777
Test: atest ArtServiceTests
Ignore-AOSP-First: ART Services.
Change-Id: I7642ea50180f95c61e874d069cecab2a63a2dbbe
7 files changed