Add sysprop for tuning caching inactive layer

A system property, debug.sf.layer_caching_active_layer_timeout_ms, is added for user to specify milliseconds timeout to tune inactive layer heuristic.

Bug: 190228095
Test: adb root; adb shell setprop debug.sf.layer_caching_active_layer_timeout_ms 100; adb shell stop; adb shell start
Change-Id: I2607f0817af15f3a8ddcaa237b2a336c7edd315e