build: improve regsub pattern for setting buildToolsVersion

This addresses the discussion in !64
https://gitlab.com/fdroid/fdroidserver/merge_requests/64

Sometimes, buildToolsVersion is a kind of gradle macro call, and other
times it is a variable assignment.  This regsub pattern now handles both of
those cases.

tests/source-files/open-keychain/open-keychain/build.gradle

@@ -0,0 +1,48 @@
+buildscript {
+    repositories {
+        jcenter()
+    }
+
+    dependencies {
+        // NOTE: Always use fixed version codes not dynamic ones, e.g. 0.7.3 instead of 0.7.+, see README for more information
+        classpath 'com.android.tools.build:gradle:1.2.3'
+        classpath files('gradle-witness.jar')
+        // bintray dependency to satisfy dependency of openpgp-api lib
+        classpath 'com.novoda:bintray-release:0.2.7'
+        
+        classpath 'org.kt3k.gradle.plugin:coveralls-gradle-plugin:2.0.1'
+        // classpath 'com.stanfy.spoon:spoon-gradle-plugin:1.0.2'
+    }
+}
+
+allprojects {
+    repositories {
+        jcenter()
+    }
+}
+
+task wrapper(type: Wrapper) {
+    gradleVersion = '2.4'
+}
+
+subprojects {
+    tasks.withType(Test) {
+        maxParallelForks = 1
+    }
+}
+
+// Ignore tests for external spongycastle
+project(':extern:spongycastle') {
+    subprojects {
+        // Need to re-apply the plugin here otherwise the test property below can't be set.
+        apply plugin: 'java'
+        test.enabled = false
+    }
+}
+
+// SDK Version and Build Tools used by all subprojects
+// See http://tools.android.com/tech-docs/new-build-system/tips#TOC-Controlling-Android-properties-of-all-your-modules-from-the-main-project.
+ext {
+    compileSdkVersion = 22
+    buildToolsVersion = '22.0.1'
+}