Fix NPE in renameAccount

If newName is already in use, method will return requested account.
Bug: 368210916
Test: atest CtsAccountManagerTestCases
Flag: EXEMPT bugfix
Change-Id: I5deb176e1baf7b82752f52e078c18fcec7271c49

Change-Id: I0e479e86b9d8e1edd853b4c0565002d17185e1c9
1 file changed