Extract version constants into top-level build file.

build.gradle

@@ -33,3 +33,12 @@ buildscript {
 		classpath files('libs/gradle-witness.jar')
 	}
 }
+
+project.ext {
+	buildToolsVersion = '30.0.2'
+	compileSdkVersion = 30
+	minSdkVersion = 16
+	targetSdkVersion = 29
+	versionCode = 10210
+	versionName = '1.2.10'
+}