Create/delete test files from shell since app might not have access.

Fixes: 133264914
Fixes: 132992346
Test: atest cts/tests/app/src/android/app/cts/DownloadManagerTest.java
Test: atest cts/tests/app/DownloadManagerLegacyTest/src/android/app/cts/DownloadManagerLegacyTest.java
Test: atest cts/tests/app/DownloadManagerApi28Test/src/android/app/cts/DownloadManagerApi28Test.java
Change-Id: I71f19e7a074602bf2018123a3d3deb5527bf112c
4 files changed