improve the OTA progress bar

Make the OTA progress bar advance a bit more smoothly by reallocating
how much is dedicated to various phases of incremental OTA
installation, based on some crude measurements of how long each phase
takes.
diff --git a/tools/releasetools/amend_generator.py b/tools/releasetools/amend_generator.py
index 70e71d4..f8f4344 100644
--- a/tools/releasetools/amend_generator.py
+++ b/tools/releasetools/amend_generator.py
@@ -87,6 +87,10 @@
     'dur' seconds."""
     self.script.append("show_progress %f %d" % (frac, int(dur)))
 
+  def SetProgress(self, frac):
+    """Not implemented in amend."""
+    pass
+
   def PatchCheck(self, filename, *sha1):
     """Check that the given file (or MTD reference) has one of the
     given *sha1 hashes."""