Make Hardware class platform specific.

The Hardware class abstracts the interaction with the actual hardware
which is inherently platform-specific. This patch implementes a stub
version for Android and moves the Chromium OS implementation, handling
both with the same public factory method.

Bug: 23084776
TEST=emerge-link update_engine; `mma -i` builds the stubs.

Change-Id: I19c791a2e3fa22f0ac38bbe68a9c47838bcf019a
diff --git a/update_engine.gyp b/update_engine.gyp
index 0f0a770..5d1ce97 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -173,7 +173,7 @@
         'file_descriptor.cc',
         'file_writer.cc',
         'filesystem_verifier_action.cc',
-        'hardware.cc',
+        'hardware_chromeos.cc',
         'http_common.cc',
         'http_fetcher.cc',
         'hwid_override.cc',