psi support for low memory detection inside ActivityManagerService

Add LowMemDetector class and use it in ActivityManagerService to detect
memory conditions based on in-kernel psi monitors.

Test: boots, detects low memory
Bug: 129476847
Change-Id: I7a05ba36196d24ce5147b97c076e38b92dc71e2e
Signed-off-by: Tim Murray <timmurray@google.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>