UI polish/tweaks for blocked numbers UI.

- Update the strings as in the mocks
- Change the theme on the dialogue to match (mock)
- Change the hint text to "Phone number"
- Change the keyboard to the phone keyboard
- Upon user clicking "Add a number," put focus on the text field and have keyboard popped up

Bug: 26917087
Change-Id: I84320095050ddf808ae89c9800f94d1dc05089b5