Move build_tools/sdk version to main gralde

- Helps avoiding issues between submodules and travis setup.
- It works only if all modules uses the same dependencies.

Habitica/build.gradle

@@ -138,8 +138,8 @@ dependencies {
 }
 
 android {
-    compileSdkVersion 26
-    buildToolsVersion "26.0.1"
+    compileSdkVersion sdk_version
+    buildToolsVersion build_tools_version
 
     defaultConfig {
         applicationId "com.habitrpg.android.habitica"
@@ -181,6 +181,7 @@ android {
     productFlavors {
         dev {
             minSdkVersion 21
+            buildConfigField "String", "STAGING_KEY", "\"1dcf9ed3-3b14-45b3-9e7e-acdfff68a368\""
         }
 
         prod {

build.gradle

@@ -2,6 +2,9 @@
 
 buildscript {
     ext.kotlin_version = '1.1.51'
+    ext.build_tools_version = '26.0.1'
+    ext.sdk_version = 26
+
     repositories {
         jcenter()
         maven { url "https://plugins.gradle.org/m2/" }

seeds-sdk/build.gradle

@@ -28,9 +28,8 @@ repositories {
 }
 
 android {
-    compileSdkVersion 25
-
-    buildToolsVersion "25.0.3"
+    compileSdkVersion sdk_version
+    buildToolsVersion build_tools_version
 
     defaultConfig {
         minSdkVersion 14