CtsDropBoxManagerTestCases not_secondary_user

Marks this CTS test module as NOT supporting secondary_user, meaning
that the tests are not eligible to be run from a regular Android user
other than the system user (i.e. other than user 0).

This module already uses SwitchUserTargetPreparer, meaning that it
explicitly needs to switch to system user first.

Note that 'user' here refers to the concept of users in Android
Multiuser; it is NOT the same as a uid.

Bug: 141773058
Test: disabled for secondary user
Change-Id: I130aaf540c9fe1775f368447c29cd85bade5dc43