Move IsBootDeviceRemovable() to the HardwareInterface.

The UpdateCheckScheduler class had a IsBootDeviceRemovable method to
mock out the value during testing of that class. Instead, this patch
uses the HardwareInterface to mock out that value.

BUG=chromium:358269
TEST=Unittests updated.

Change-Id: Ib20f70fa0468aaa4bc8bb1b674084bd9a61e5085
Reviewed-on: https://chromium-review.googlesource.com/197598
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/hardware.h b/hardware.h
index fdcdf13..99c0803 100644
--- a/hardware.h
+++ b/hardware.h
@@ -20,6 +20,7 @@
   // HardwareInterface methods.
   virtual std::string BootKernelDevice() const override;
   virtual std::string BootDevice() const override;
+  virtual bool IsBootDeviceRemovable() const override;
   virtual std::vector<std::string> GetKernelDevices() const override;
   virtual bool IsKernelBootable(const std::string& kernel_device,
                                 bool* bootable) const override;