cmd/gomobile: upgrade minimal javac version to 1.8

Fixes golang/go#59803

upgrade minimal javac for gendex as well

update doc related javac version info

fix test failure

Author: SuzukiHonoka

cmd/gomobile/bind.go

@@ -44,7 +44,7 @@ example, in Android Studio (1.2+), an AAR file can be imported using
 the module import wizard (File > New > New Module > Import .JAR or
 .AAR package), and setting it as a new dependency
 (File > Project Structure > Dependencies).  This requires 'javac'
-(version 1.7+) and Android SDK (API level 16 or newer) to build the
+(version 1.8+) and Android SDK (API level 16 or newer) to build the
 library for Android. The ANDROID_HOME and ANDROID_NDK_HOME environment
 variables can be used to specify the Android SDK and NDK if they are
 not in the default locations. Use the -javapkg flag to specify the Java

cmd/gomobile/bind_androidapp.go

@@ -245,7 +245,7 @@ func buildAAR(srcDir, androidDir string, pkgs []*packages.Package, targets []tar
 }
 
 const (
-	javacTargetVer = "1.7"
+	javacTargetVer = "1.8"
 	minAndroidAPI  = 16
 )
 

cmd/gomobile/bind_test.go

@@ -190,7 +190,7 @@ GOOS=android CGO_ENABLED=1 gobind -lang=go,java -outdir=$WORK{{if .JavaPkg}} -ja
 mkdir -p $WORK/src-android-arm
 PWD=$WORK/src-android-arm GOMODCACHE=$GOPATH/pkg/mod GOOS=android GOARCH=arm CC=$NDK_PATH/toolchains/llvm/prebuilt/{{.NDKARCH}}/bin/armv7a-linux-androideabi16-clang CXX=$NDK_PATH/toolchains/llvm/prebuilt/{{.NDKARCH}}/bin/armv7a-linux-androideabi16-clang++ CGO_ENABLED=1 GOARM=7 GOPATH=$WORK:$GOPATH go mod tidy
 PWD=$WORK/src-android-arm GOMODCACHE=$GOPATH/pkg/mod GOOS=android GOARCH=arm CC=$NDK_PATH/toolchains/llvm/prebuilt/{{.NDKARCH}}/bin/armv7a-linux-androideabi16-clang CXX=$NDK_PATH/toolchains/llvm/prebuilt/{{.NDKARCH}}/bin/armv7a-linux-androideabi16-clang++ CGO_ENABLED=1 GOARM=7 GOPATH=$WORK:$GOPATH go build -x -buildmode=c-shared -o=$WORK/android/src/main/jniLibs/armeabi-v7a/libgojni.so ./gobind
-PWD=$WORK/java javac -d $WORK/javac-output -source 1.7 -target 1.7 -bootclasspath {{.AndroidPlatform}}/android.jar *.java
+PWD=$WORK/java javac -d $WORK/javac-output -source 1.8 -target 1.8 -bootclasspath {{.AndroidPlatform}}/android.jar *.java
 jar c -C $WORK/javac-output .
 `))
 

cmd/gomobile/gendex.go

@@ -74,8 +74,8 @@ func gendex() error {
 	}
 	cmd := exec.Command(
 		"javac",
-		"-source", "1.7",
-		"-target", "1.7",
+		"-source", "1.8",
+		"-target", "1.8",
 		"-bootclasspath", platform+"/android.jar",
 		"-d", tmpdir+"/work",
 	)