cmd/gomobile: bump minimum sdk version and update gradle for NDK 18

NDK 18 drops support for API levels below 16. Bump it to 16.

NDK 18 also needs a newer version of the Android gradle plugin.
Update the bind example and test build.gradle files.

Change-Id: I71499c88b48c43b2d4da50e415cb0b0dcbbbed75
Reviewed-on: https://go-review.googlesource.com/136775
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/bind/java/seq_test.go b/bind/java/seq_test.go
index 05feed6..3f5de90 100644
--- a/bind/java/seq_test.go
+++ b/bind/java/seq_test.go
@@ -195,27 +195,30 @@
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.0.1'
+        classpath 'com.android.tools.build:gradle:3.1.0'
     }
 }
 
 allprojects {
-    repositories { jcenter() }
+    repositories {
+		google()
+		jcenter()
+	}
 }
 
 apply plugin: 'com.android.library'
 
 android {
     compileSdkVersion 'android-19'
-    buildToolsVersion '21.1.2'
-    defaultConfig { minSdkVersion 15 }
+    defaultConfig { minSdkVersion 16 }
 }
 
 repositories {
     flatDir { dirs 'libs' }
 }
+
 dependencies {
-    compile(name: "pkg", ext: "aar")
+    implementation(name: "pkg", ext: "aar")
 }
 `
 
diff --git a/cmd/gomobile/env.go b/cmd/gomobile/env.go
index f2ec576..471b009 100644
--- a/cmd/gomobile/env.go
+++ b/cmd/gomobile/env.go
@@ -272,7 +272,7 @@
 	"arm": {
 		arch:       "arm",
 		abi:        "armeabi-v7a",
-		platform:   "android-15",
+		platform:   "android-16",
 		gcc:        "arm-linux-androideabi-4.9",
 		toolPrefix: "arm-linux-androideabi",
 	},
@@ -287,7 +287,7 @@
 	"386": {
 		arch:       "x86",
 		abi:        "x86",
-		platform:   "android-15",
+		platform:   "android-16",
 		gcc:        "x86-4.9",
 		toolPrefix: "i686-linux-android",
 	},
diff --git a/cmd/gomobile/init_test.go b/cmd/gomobile/init_test.go
index b883b76..c0180ce 100644
--- a/cmd/gomobile/init_test.go
+++ b/cmd/gomobile/init_test.go
@@ -115,9 +115,9 @@
 mkdir -p $GOMOBILE
 WORK={{.GOPATH}}/pkg/gomobile/work
 go install -x golang.org/x/mobile/cmd/gobind
-PWD=$NDK_PATH $NDK_PATH/prebuilt/{{.GOOS}}-{{.NDKARCH}}/bin/python2.7 build/tools/make_standalone_toolchain.py --arch=arm --api=15 --install-dir=$GOMOBILE/ndk-toolchains/arm
+PWD=$NDK_PATH $NDK_PATH/prebuilt/{{.GOOS}}-{{.NDKARCH}}/bin/python2.7 build/tools/make_standalone_toolchain.py --arch=arm --api=16 --install-dir=$GOMOBILE/ndk-toolchains/arm
 PWD=$NDK_PATH $NDK_PATH/prebuilt/{{.GOOS}}-{{.NDKARCH}}/bin/python2.7 build/tools/make_standalone_toolchain.py --arch=arm64 --api=21 --install-dir=$GOMOBILE/ndk-toolchains/arm64
-PWD=$NDK_PATH $NDK_PATH/prebuilt/{{.GOOS}}-{{.NDKARCH}}/bin/python2.7 build/tools/make_standalone_toolchain.py --arch=x86 --api=15 --install-dir=$GOMOBILE/ndk-toolchains/x86
+PWD=$NDK_PATH $NDK_PATH/prebuilt/{{.GOOS}}-{{.NDKARCH}}/bin/python2.7 build/tools/make_standalone_toolchain.py --arch=x86 --api=16 --install-dir=$GOMOBILE/ndk-toolchains/x86
 PWD=$NDK_PATH $NDK_PATH/prebuilt/{{.GOOS}}-{{.NDKARCH}}/bin/python2.7 build/tools/make_standalone_toolchain.py --arch=x86_64 --api=21 --install-dir=$GOMOBILE/ndk-toolchains/x86_64
 cp $OPENAL_PATH/include/AL/al.h $GOMOBILE/include/AL/al.h
 mkdir -p $GOMOBILE/include/AL
diff --git a/example/bind/android/build.gradle b/example/bind/android/build.gradle
index 696b36e..973cb31 100644
--- a/example/bind/android/build.gradle
+++ b/example/bind/android/build.gradle
@@ -9,7 +9,7 @@
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.0.1'
+        classpath 'com.android.tools.build:gradle:3.1.0'
     }
 }