Make AccountManagerServiceTest work again

It was broken for quite a while. Introduced the following changes:
- Use mockito to mock AppOpsManager and UserManager
- Use MockContext directly and override what is necessary.
  RenamingDelegatingContext doesn't work with absolute file names for
  database paths, which is the case for system server DBs.

Bug: 26279712
Change-Id: I9f2a3892f9bd7a882d78f0bb7beb8669f7dc8c6f