[Gradle] Allow dialer to be compiled as a library

This CL adds a new build-library.gradle file for AOSP dialer.
This allows the dialer to be built as a library that can be
included from GoogleDialer.

Switching to a library project meant making two other changes:
  - changed all switch statements that used resources to if statements.
    This was required because resource IDs are not final in library
    projects.
  - changed InCalUI code to import com.android.dialer.R instead of
    com.android.incallui.R. See http://b.android.com/82743 for more info
    on why this is required.

src-N isn't supported yet. Also, this isn't the ideal project layout.
In the future we should consider switching to the following layout:
  - dialer/incallui/ <- incall UI as an independent library project
  - dialer/dialerlib/ <- dialer code as an independent library project
  - dialer/app <- skelent app that builds a standalone dialer AOSP app

Bug: 26676586
Change-Id: I07fbee4d33cc683539e4f8b3953c93f1427af9d7