lib/utils.c: add mutex to get_psched_settings()

Let's add a mutex to get_psched_settings() hoping to solve worst
case scenarios when calling get_psched_settings() from multiple
threads.

Also, only read the environment variables once, who knows whether
they are modified concurrently.

(cherry picked from commit 24d669a3a990d1d637fc94698cc4ae2c6e8e178e)

Test: Compiles and ueventd proc_net denial is gone.
Bug: 35197529
Change-Id: Ic49dd28d3ab951a4d1b557a5e27290366f4640cc