Convert buildSrc to kotlin

buildSrc/build.gradle.kts

...roject/briar/desktop/builddata/AbstractBuildDataTask.java

...briarproject/briar/desktop/builddata/BuildDataPlugin.java

...ect/briar/desktop/builddata/BuildDataPluginExtension.java

...org/briarproject/briar/desktop/builddata/FileBuilder.java

.../briar/desktop/builddata/GenerateBuildDataSourceTask.java

...n/java/org/briarproject/briar/desktop/builddata/Util.java

...rproject/briar/desktop/builddata/AbstractBuildDataTask.kt

...g/briarproject/briar/desktop/builddata/BuildDataPlugin.kt

...oject/briar/desktop/builddata/BuildDataPluginExtension.kt

...n/org/briarproject/briar/desktop/builddata/FileBuilder.kt

...ct/briar/desktop/builddata/GenerateBuildDataSourceTask.kt

...n/kotlin/org/briarproject/briar/desktop/builddata/Util.kt