[mapplauncherd] Put booster sockets under XDG_RUNTIME_DIR

This is a compatibility-breaking change with boosters. The socketId
field is now expected to be a socket name, not a path. Sockets are
placed under ${XDG_RUNTIME_DIR}/mapplauncherd/, defaulting to /tmp/.

Invoker currently expects sockets in that path and named 'booster-X'
where X is the single character booster type. That is expected to
change when types are refactored to use full strings.