Fix ActionDisabledByAdminDialogHelperTest by setting a mock SupervisionManager as system service.

A shadow of DPM is automatically added as system service by Robolectric infra. Since SupervisionManager is new, it doesn't have that.

We will consider changing Robolectric to do the same thing when SupervisionManager becomes more stable.

Flag: TEST_ONLY
Bug: 382034839
Test: atest ActionDisabledByAdminDialogHelperTest
Change-Id: I389938df10ce97c563511473b0aeaee52225f419