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
Merged-In: I6a0f13d5c71ab5529a76751f68ac0f15834323d1
diff --git a/init/Android.mk b/init/Android.mk
index 207b5e7..b49fb3b 100644
--- a/init/Android.mk
+++ b/init/Android.mk
@@ -48,6 +48,7 @@
 include $(CLEAR_VARS)
 LOCAL_CPPFLAGS := $(init_cflags)
 LOCAL_SRC_FILES := \
+    block_dev_initializer.cpp \
     devices.cpp \
     first_stage_init.cpp \
     first_stage_main.cpp \