[hwkey] Add HWKEY_ERR_ALREADY_EXISTS

The opaque key handler in hwkey needs an error code to indicate that an
opaque handle has already been generated and is still in use. Add the
HWKEY_ERR_ALREADY_EXISTS error code for this case.

Test: build.py
Bug: 185483587
Change-Id: Ic287734893a3ce513a439b2b68a270c2f13b5ce5