Improve styling of schema code samples dialog

I adopted Krinkle's suggestions (from bug 52054) more or less verbatim:

- Specify min-width instead of width.
- Make box-shadow less obnoxious
- Add 1px border.
- No rounded corners

Bug: 52054
Change-Id: I2966f2332a1690288b6adacf8a837aa4b2d8325f
Merge "Improve styling of schema code samples dialog"