nfc(api): Restore behavior from Android T (follow-up)

Previous bug fix in ag/23755172 had a bug, we should always return null
if the service is not available regardless of |sIsInitialized| state.

Bug: 296358872
Test: Manual tests
Change-Id: I97a3db94189cc3b7bbecd8f25adb42c691fbdaa8
1 file changed