Use single-argument form of firstKeyOf()

Use single-argument form of firstKeyOf()

A few other clean-ups in the affected files:
* use plain String concatenation instead of StringBuilder where appropriate
* simplify single-use return variables
* <> operator
* avoid calling new String()

Change-Id: I3e1891509f21211c489cabb64868d78e4dddec10
Signed-off-by: Stephen Kitt <skitt@redhat.com>