Explicitly exit from the am instrument after test completes

To speed up the test. For a trivial am instrument run:

[Before]
0m01.53s real     0m00.16s user     0m00.16s system

[After]
0m00.72s real     0m00.21s user     0m00.11s system

Bug: 204195830
Test: am instrument -w
Change-Id: I66196d1db9169681dabb2e5dacdd18e6105ad75a