Dialer view facelift

The current dialer screen layout has a few issues:

• The delete button is at the top, which is nice semantically but not ideal ergonomically
• The button being next to the phone number entry means it can hold fewer digits, and also makes it look a bit off-balance
• The grid of rectangular buttons is a bit heavy visually. @sam.hewitt played with round buttons in a web mockup, which would help a lot with this
• The phone number entry uses the regular UI font size, which makes it look quite small for something so important. Other phone OSes use significantly larger sizes here.
• The phone number looks a bit odd left-aligned

Some suggestions to make it nicer:

• Centering the phone number
• Making the phone number larger
• Moving the delete button to the bottom
• Rounding all buttons, and making the grid buttons circular
• Making the call button a bit less wide

cc @sam.hewitt