Restrict testEnableProtection_secondaryUser_throws to non-admin users.

The test `testEnableProtection_secondaryUser_throws` is intended to
verify behavior for non-admin users. This change adds an assumption to
ensure the test only runs when the current user is not an admin, for
compatibility with HSUM devices.

Bug: 416134896
Test: atest AdvancedProtectionManagerTest on different device types
Flag: EXEMPT TEST_ONLY
Change-Id: I71de70951b436ecf26b5c4e5dbccb3d506a66967