Add lmkd_native.psi_partial_stall_ms test config

With MGLRU enabled, we see 5% - 16% fewer LMKs from the Beta population
compared to a non-MGLRU baseline. We'll try decreasing the LMKD wakeup
threshold by 20% in one of the experiment configurations to see if it
results in a comparable number of kills to non-MGLRU.

Bug: 384577842
Ignore-AOSP-First: 25Q2 Beta Experiment only go/android-memcgv2-exp
Change-Id: I566a4b294182ece0bd586c88e4f754160235d16f