featherless 2414525074
[Buttons] Replace MDCFlatButton with MDCButton + text themer in content edge insets example. (#3583)
Also fixes a bug where the button was being initialized as a system button instead of as a custom button. This was affecting the highlighted text state.

Pivotal story: https://www.pivotaltracker.com/story/show/157189341

Before (normal / highlighted):
![simulator screen shot - iphone se - 2018-04-30 at 09 21 34](https://user-images.githubusercontent.com/45670/39429179-18130c08-4c58-11e8-972a-b96dd827ee26.png) ![simulator screen shot - iphone se - 2018-04-30 at 09 21 36](https://user-images.githubusercontent.com/45670/39429181-193b0ff4-4c58-11e8-8dab-dcdfcf86a7cf.png)

After (normal / highlighted):
![simulator screen shot - iphone se - 2018-04-30 at 09 21 03](https://user-images.githubusercontent.com/45670/39429211-2e171710-4c58-11e8-95e6-07eb614857c8.png) ![simulator screen shot - iphone se - 2018-04-30 at 09 21 04](https://user-images.githubusercontent.com/45670/39429215-30bde2c8-4c58-11e8-882a-e585da609684.png)
2018-04-30 12:48:38 -04:00
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