main: refactor theme loading code to reduce duplication

main: refactor theme loading code to reduce duplication

Right now we read /usr/share/plymouthd/plymouthd.defaults
and /etc/plymouth/plymouthd.conf to find the configured splash
screen.  The two functions that do this are basically copy-and-paste
of each other.

This commits splits some of the duplicated code out into a common
function for clarity.  Doing this also helps to facilitate adding
more configuration options, which we'll need to do in the future
to support a global start up delay.