Make the bg activity start exemption app op available to tests.

Mark OPSTR_SYSTEM_EXEMPT_FROM_ACTIVITY_BG_START_RESTRICTION as a TestApi
so that we can use it in tests.

Bug: 414682995
Test: atest ActivityManagerTest#testAddOnUidImportanceListener_legacy
Flag: EXEMPT exposing API for test only
Change-Id: I9aba67f29fa5a69dd348958913c2470dfec320b5