AUPT adding media profile test

MediaStressTest uses Downloads app to launch a file placed in the Download directory on the Internal Storage Drive. Currently supports launching audio and video files.

General Framework addition: Adding AbstractDownloadsHelper and DownloadsHelperImpl to control Downloads application.

Bug: 30755418
Change-Id: Ia758abdafb041ada262d2e15730f9dc0ea31f1e2
Fixes:
Test: make downloads-app-helper
Test: make media-profile-test
Test: make  google_tests
Test: Example ./test-setup.sh -s HT6760200022 -p media -e media-type video -e loop true -e playtime 60 -l filename /google/data/ro/teams/tradefed/testdata/media/multimeter/video_720p_60fps_4min_mountham.mp4 -i 30 google_tests.zip