DO NOT MERGE Switch to system user before running CtsOsTestCases module.

StorageManagerTest fails when being run in secondary user. Skip mount obb
tests from running in secondary, since that feature only works in system user.

The code has been refactored in Q+ and the test passes in post P branches -
so do not merge downstream.

Fixes: 134634827
Test: atest CtsOsTestCases
Change-Id: Iad4285e56dbb104a930c33d66021eb4b30f37e09