Fix the Platform_sdk_version default.

N has merged into aosp now.

Test: OUT_DIR=ndk-out DIST_DIR=ndk-dist \
          build/soong/scripts/build-ndk-prebuilts.sh
Bug: None

Change-Id: I2685c700e2cb138708a3e8a5f51281c30fb582db
diff --git a/android/variable.go b/android/variable.go
index 5544c12..c1948b0 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -137,7 +137,7 @@
 
 func (v *productVariables) SetDefaultConfig() {
 	*v = productVariables{
-		Platform_sdk_version:       intPtr(23),
+		Platform_sdk_version:       intPtr(24),
 		HostArch:                   stringPtr("x86_64"),
 		HostSecondaryArch:          stringPtr("x86"),
 		DeviceName:                 stringPtr("flounder"),