BootControlStub: New class used if CreateBootControl() fail.

This can happen if running update_engine on Brillo in the emulator or
other device lacking a boot_control HAL implementation. With this
change, the update_engine process continues to run but updates won't
work.

Bug: 25012817
Change-Id: I03d516c5e9a53c893c376885cd48068cd359e902
Test: update_engine no longer crashes on brilloemulator_arm.
diff --git a/Android.mk b/Android.mk
index b91a9f4..c3b0fda 100644
--- a/Android.mk
+++ b/Android.mk
@@ -126,6 +126,7 @@
 LOCAL_SRC_FILES := \
     action_processor.cc \
     boot_control_android.cc \
+    boot_control_stub.cc \
     bzip.cc \
     bzip_extent_writer.cc \
     certificate_checker.cc \