init: Use exec_background instead of exec

extra_free_kbytes.sh sometimes takes a long time to execute, especially
on some slower devices. We don't need to wait for its completion in this
script before continuing init execution. This saves up to 200ms boot
time on some slower device models.

Bug: http://b/383225735
Test: manual, bootperf over 100 iterations

Change-Id: I19e3566d71c0e6f57045e1475d20b68bc204b5a4