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.

(cherry picked from commit fb905d9b8d49f8fe41297c7aba2dd0942f1be311)

Change-Id: I66f9473c10fbe8f56e9000ce3e605b2bee345db8
diff --git a/common/fake_hardware.h b/common/fake_hardware.h
index 23d6498..0bd297b 100644
--- a/common/fake_hardware.h
+++ b/common/fake_hardware.h
@@ -62,6 +62,18 @@
 
   int GetPowerwashCount() const override { return powerwash_count_; }
 
+  bool SchedulePowerwash() override {
+    powerwash_scheduled_ = true;
+    return true;
+  }
+
+  bool CancelPowerwash() override {
+    powerwash_scheduled_ = false;
+    return true;
+  }
+
+  bool IsPowerwashScheduled() { return powerwash_scheduled_; }
+
   bool GetNonVolatileDirectory(base::FilePath* path) const override {
     return false;
   }
@@ -115,6 +127,7 @@
   std::string firmware_version_;
   std::string ec_version_;
   int powerwash_count_;
+  bool powerwash_scheduled_{false};
 
   DISALLOW_COPY_AND_ASSIGN(FakeHardware);
 };