KeyHandler: Integrate in tri-state-key handling via UEventObserver API

This basically adds required code to support both switch and extcon
based tri-state-key. This also adds tri-state-key-calibrate script for
devices that need to be calibrated on boot (OnePlus7+)

Change-Id: I2e6c5d2861569750bead05edacf6e328a5227077

KeyHandler/tri-state-key-calibrate.rc

@@ -0,0 +1,9 @@
+service vendor.tri-state-key-calibrate /vendor/bin/tri-state-key-calibrate
+    class main
+    user system
+    group system
+    oneshot
+    disabled
+
+on post-fs-data
+    start vendor.tri-state-key-calibrate