Settings UI elements for importing VM to block list.

+ Hidden for now, and no logic implemented yet.
+ Add UI elements for importing send-to-voicemail settings into
Dialer's blocked calls list.
+ Add strings for these settings.
+ Rearrange some dimens/colors/styles.

+ Add blue button style to theme as the default button.
+ Added FlatButtonStyle to make it easier to style standard
borderless buttons.

Bug: 23351616
Change-Id: I21804ed4f8498008f0e79d7b8c4f2b12a9a2f914