在gradle的buildTypes中对变量进行引用
buildConfigField 'int', 'DESIGN_WIDTH', "${project.ext.designWidth}"
buildConfigField 'int', 'DESIGN_HEIGHT', "${project.ext.designHeight}"
buildConfigField 'int', 'DESIGN_WIDTH', "${project.ext.designWidth}"
buildConfigField 'int', 'DESIGN_HEIGHT', "${project.ext.designHeight}"