Avoid the suppression of an errorprone warning

Lock mService.mSpManager in initializeStorageWithCredential() so that
GuardedBy warnings don't have to be suppressed in that method anymore.

Test: m FrameworksServicesTests RUN_ERROR_PRONE=true
Test: atest LockSettingsServiceTests
Change-Id: I7451a7d32c3feb6c1ac40371cdb6f24f7b1fdd6d