in amend, remove symlink targets before creating them

amend's symlink command isn't idempotent, so if you restart after the
symlinks have been completed then the second installation attempt will
fail.  Work around this by deleting all symlink targets before
creating symlinks.

Bug: 2020011

tools/releasetools/amend_generator.py

@@ -180,6 +180,7 @@ class AmendGenerator(object):
 
   def MakeSymlinks(self, symlink_list):
     """Create symlinks, given a list of (dest, link) pairs."""
+    self.DeleteFiles([i[1] for i in symlink_list])
     self.script.extend(["symlink %s %s" % (i[0], self._FileRoot(i[1]))
                         for i in sorted(symlink_list)])