Fix potential NPE in SapService

It could throw NPE if mService is being cleaned up
while accessing the variable.

This also fixes SapServiceTest failed in the local enviornment.

Flag: EXEMPT, strict null check
Bug: 339695797
Test: atest SapServiceTest
Change-Id: Ib2b09cad6ff84f41e66f07cfb066bc33e3ad1c0c