Properties: move mService to final

The AdapterService is valid during the whole lifetime of
AdapterProperties. Putting it final reflect this and prevent any null
pointer exception that happen in monkey test

Bug: 328842207
Fix: 328842207
Test: None
Flag: Exempt refactor
Change-Id: I75f3210a95df24e511c48ceb5a594cce4e7e7f4f