Bug 1627354: Part 2 - Update gradle files to set buildToolsVersion; r=nalexander

Per https://developer.android.com/studio/releases/build-tools, the default
version of the build tools used is internal to the Gradle plugin for Android,
which in the current version of Gradle is 28. To use 29, we need to explicitly
specify that in `.gradle` files using `buildToolsVersion`.

We also change the specification of `compileSdkVersion` to obtain that
information from the build configuration.

Differential Revision: https://phabricator.services.mozilla.com/D69632

--HG--
extra : moz-landing-system : lando

build.gradle

@@ -15,7 +15,8 @@ allprojects {
         topsrcdir = gradle.mozconfig.topsrcdir
         topobjdir = gradle.mozconfig.topobjdir
 
-        compileSdkVersion = 28
+        buildToolsVersion = mozconfig.substs.ANDROID_BUILD_TOOLS_VERSION
+        compileSdkVersion = tryInt(mozconfig.substs.ANDROID_TARGET_SDK)
         targetSdkVersion = tryInt(mozconfig.substs.ANDROID_TARGET_SDK)
         minSdkVersion = tryInt(mozconfig.substs.MOZ_ANDROID_MIN_SDK_VERSION)
         manifestPlaceholders = [

mobile/android/examples/messaging_example/app/build.gradle

@@ -5,6 +5,7 @@ apply plugin: 'com.android.application'
 apply from: "${topsrcdir}/mobile/android/gradle/product_flavors.gradle"
 
 android {
+    buildToolsVersion project.ext.buildToolsVersion
     compileSdkVersion project.ext.compileSdkVersion
 
     compileOptions {

mobile/android/examples/port_messaging_example/app/build.gradle

@@ -5,6 +5,7 @@ apply plugin: 'com.android.application'
 apply from: "${topsrcdir}/mobile/android/gradle/product_flavors.gradle"
 
 android {
+    buildToolsVersion project.ext.buildToolsVersion
     compileSdkVersion project.ext.compileSdkVersion
 
     compileOptions {
@@ -46,4 +47,4 @@ dependencies {
     implementation 'com.android.support.constraint:constraint-layout:1.1.3'
     testImplementation 'junit:junit:4.12'
     implementation project(path: ':geckoview')
-}
\ No newline at end of file
+}

mobile/android/geckoview/build.gradle

@@ -60,6 +60,7 @@ def getBuildId() {
 }
 
 android {
+    buildToolsVersion project.ext.buildToolsVersion
     compileSdkVersion project.ext.compileSdkVersion
 
     useLibrary 'android.test.runner'

mobile/android/geckoview_example/build.gradle

@@ -5,6 +5,7 @@ apply plugin: 'com.android.application'
 apply from: "${topsrcdir}/mobile/android/gradle/product_flavors.gradle"
 
 android {
+    buildToolsVersion project.ext.buildToolsVersion
     compileSdkVersion project.ext.compileSdkVersion
 
     defaultConfig {