Decouple presubmit tests for shortcut manager from file system.

As the first step of re-enabling previously disabled presubmit tests in
ShortcutManager, this CL decouples the tests from file system to avoid
breakage due to incompliance issues between test framework and file util
implementation.

Bug: 389780499
Test: manual
Flag: TEST_ONLY
Change-Id: I6158747868506fd32642f4c0935db96ae75a09a9