Support unnecessary parentheses

This CL tweaks the CallingIdentityTokenDetector to handle
the case where there are extra (unnecessary) parentheses
present in the code, such as in
   final long token = (clearCallingIdentity());

This isn't very likely, but the next version of lint automatically
runs all lint unit tests through extra checks (inserting parentheses,
whitespace, replacing imported names with fully qualified names etc
to help detectors shake out corner cases they may not have tested
for but probably want their detectors to catch.

Test: Existing
Bug: None
Change-Id: I5af15b2ff4c13d3361a368faecdf6ef23815f6c8