Implement powerwash on Android.

Powerwash, the name for the equivalent of a factory reset or /data wipe,
can be triggered in Android by writing the desired command to the
recovery command file and rebooting into recovery.

This patch moves the powerwash scheduling/canceling logic to the
HardwareInterface and implements it on Android.

Bug: 28700985
TEST=Called update_engine_client passing POWERWASH=1, BCB is stored up
to offset 832.

Change-Id: If737fd4b9b3e2ed9bce709b3b59f22e9f0a3dc9a
diff --git a/common/constants.h b/common/constants.h
index 660d9a9..649034e 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -19,13 +19,6 @@
 
 namespace chromeos_update_engine {
 
-// The name of the marker file used to trigger powerwash when post-install
-// completes successfully so that the device is powerwashed on next reboot.
-extern const char kPowerwashMarkerFile[];
-
-// The contents of the powerwash marker file.
-extern const char kPowerwashCommand[];
-
 // Directory for AU prefs that are preserved across powerwash.
 extern const char kPowerwashSafePrefsSubDirectory[];
 
@@ -100,6 +93,7 @@
 extern const char kPayloadPropertyMetadataHash[];
 extern const char kPayloadPropertyAuthorization[];
 extern const char kPayloadPropertyUserAgent[];
+extern const char kPayloadPropertyPowerwash[];
 
 // A download source is any combination of protocol and server (that's of
 // interest to us when looking at UMA metrics) using which we may download