Add boot animation progress system property.

Test: m
Bug: 175686819
Change-Id: I0504a6aa94da9f16efba6e97beb789f3ff1a578a
diff --git a/services/surfaceflinger/StartPropertySetThread.cpp b/services/surfaceflinger/StartPropertySetThread.cpp
index db82772..f42cd53 100644
--- a/services/surfaceflinger/StartPropertySetThread.cpp
+++ b/services/surfaceflinger/StartPropertySetThread.cpp
@@ -31,6 +31,7 @@
     property_set(kTimestampProperty, mTimestampPropertyValue ? "1" : "0");
     // Clear BootAnimation exit flag
     property_set("service.bootanim.exit", "0");
+    property_set("service.bootanim.progress", "0");
     // Start BootAnimation if not started
     property_set("ctl.start", "bootanim");
     // Exit immediately