CPULimiter: Refactor class to manage the CPU limitation.

This new class replaces the functionality embedded in UpdateAttempter
that limits the max CPU usage allowed by update_engine. This refactor
helps reusing this class outside of the brillo UpdateAttempter.

Bug: None
TEST=FEATURES=test emerge-link update_engine

Change-Id: Ib5487d314846b497a44bb78a3b94609571e0fe38
diff --git a/Android.mk b/Android.mk
index 04ecfe5..fb5af1b 100644
--- a/Android.mk
+++ b/Android.mk
@@ -152,6 +152,7 @@
     common/certificate_checker.cc \
     common/clock.cc \
     common/constants.cc \
+    common/cpu_limiter.cc \
     common/hash_calculator.cc \
     common/http_common.cc \
     common/http_fetcher.cc \