Defer PSS collection while an activity is starting

Make sure not to process any requested PSS collections for a brief time
after an activity start is initiated, to reduce resource contention
during the launch.

Device config atom to configure:

  ActivityManager/activity_start_pss_defer

Milliseconds; defaults to zero (PSS deferral policy disabled)

Bug: 133433769
Test: manual
Change-Id: Icf5cf2e3dde0bd5fd3ab9c4a85adfb1f3382057f