Add proper UI tests.

[miguelandres]

Remixer UI's test coverage is abysmal.

We need to do the following:
-Test RemixerItemWidgetHelper making sure that it always returns the expected layout.
-Add proper UI tests using espresso for each of the widgets. They may be hard to initialize but preferably these unit tests need to be completely independent of Remixer itself and only depend directly on a RemixerItem and and a RemixerItemWidget.
-Add UI tests to the example app, these will act as integration tests.

• I have worked on this before but it is hard, I have to say.
• Remixer fragment is in a separate window and it contains a RecyclerView, recycler views don't work with Espresso's default onData, so something else may be needed.
• Scrolling the recycler view in the test may be an issue.
• There is no support for spinners (ItemListWidget) on Espresso so you have to roll your own.
• Espresso is just ugly, is there nothing else? (I think not) I find myself constantly frustrated by this. I am just ranting by now, time to just submit the issue and stop typing.