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.cc b/common/constants.cc
index c0bb874..0ac22f3 100644
--- a/common/constants.cc
+++ b/common/constants.cc
@@ -18,11 +18,6 @@
 
 namespace chromeos_update_engine {
 
-const char kPowerwashMarkerFile[] =
-    "/mnt/stateful_partition/factory_install_reset";
-
-const char kPowerwashCommand[] = "safe fast keepimg reason=update_engine\n";
-
 const char kPowerwashSafePrefsSubDirectory[] = "update_engine/prefs";
 
 const char kPrefsSubDirectory[] = "prefs";
@@ -97,5 +92,6 @@
 const char kPayloadPropertyMetadataHash[] = "METADATA_HASH";
 const char kPayloadPropertyAuthorization[] = "AUTHORIZATION";
 const char kPayloadPropertyUserAgent[] = "USER_AGENT";
+const char kPayloadPropertyPowerwash[] = "POWERWASH";
 
 }  // namespace chromeos_update_engine