Fix flaky test cases and OEM device failed test cases

1. Add NPE check on receiver and fix some logic to fix some flaky tests.
2. Use DEVICE_NAME rather than Build.MODEL to get storage root name.

Bug: 143499864
Test: atest DocumentsUIGoogleTests
Change-Id: I71d934c593560a7822d6fe7523b92c227c738e22
Merged-In: I71d934c593560a7822d6fe7523b92c227c738e22
2 files changed