init: Factor out first-stage uevent handling into a separate class.

This will make it possible to re-use in second-stage init.

Bug: 150315914
Test: first-stage init boots
Change-Id: I6a0f13d5c71ab5529a76751f68ac0f15834323d1
diff --git a/init/Android.bp b/init/Android.bp
index 72a7bfe..17e4adf 100644
--- a/init/Android.bp
+++ b/init/Android.bp
@@ -36,6 +36,7 @@
     "util.cpp",
 ]
 init_device_sources = [
+    "block_dev_initializer.cpp",
     "bootchart.cpp",
     "builtins.cpp",
     "devices.cpp",