benchmark: use getExternalFilesDir() to save results

Instead of getFilesDir(), use getExternalFilesDir() so that results
can be accessed using adb on a production build as well

Log the paths to files where results are saved

Also update AndroidTest.xml to not uninstall apk after the test as
results need to be preserved

Bug: 147247596
Test: atest com.android.media.benchmark.tests
     on a device with factory image

Change-Id: I78e447ab0cba89cd56e72f56562a8e50036e94a0