Fetch the main user id before using it

The test was using main user id without initializing it. This should
work fine on non-HSUM because it's default-initialized to 0, but it will
break on HSUM.

While at it, rename primary -> main.

Bug: 391793604
Test: atest CtsDevicePolicyManagerTestCases:com.android.cts.devicepolicy.OrgOwnedProfileOwnerTest
Flag: EXEMPT bugfix
Change-Id: Ica3fb32f0c3442821d81d52189f50fa3353a6ff7