Use <iron-input> tag instead of is="iron-input"

This is required for inputs to work correctly after the update
to Polymer 2 [1]. Without this fix, the data binding is not
working for iron-inputs.

[1] https://polymer-library.polymer-project.org/2.0/docs/upgrade#remove-type-extension-elements

Bug: Issue 12065
Change-Id: I101a15736b761366bd65e303ee1c15a4e8874744