Address flaky AccessibilitySettings test on Android

This CL is a proposed fix to a flaky test of AccessibilitySettings on
Android.

The test regularly passes, but occasionally it fails with the error
that the activity did not finish in the 10 second polling time. Here
we add an explicit call to finish the activity at the end of the test
to see if this fixes the flakiness.

AX-Relnotes: N/A
Bug: 1181640
Change-Id: Ia0f2a10d298d7cf92f4905c9c38e7d8ac66fac17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3017406
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/master@{#900622}