am: static imports ActivityManagerInternal in PSC

For consistency with OomAdjuster and OomAdjusterImpl, use static imports
for ActivityManagerInternal's constantes used in ProcessStateController.

Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Test: atest FrameworksMockingServicesTests_android_server_am_broadcast
Flag: EXEMPT pure refactor

Change-Id: Ic372c34a86e7d25945caa3d83061ee146e8a45bf