Move Ndk_abis from soong.config to soong.variables

soong.config is not cleared between builds, which can cause
problems when switching between an ndk build and a platform build.

Bug: 118398924
Test: OUT_DIR=out_ndk DIST_DIR=dist build/soong/scripts/build-ndk-prebuilts.sh
Change-Id: Ifc03a0c25f46625375cdda5723e4a09d7d8050d5
diff --git a/android/config.go b/android/config.go
index 0aee0e3..3b7b477 100644
--- a/android/config.go
+++ b/android/config.go
@@ -41,7 +41,6 @@
 // config file. These will be included in the config struct.
 type FileConfigurableOptions struct {
 	Mega_device *bool `json:",omitempty"`
-	Ndk_abis    *bool `json:",omitempty"`
 	Host_bionic *bool `json:",omitempty"`
 }
 
@@ -299,7 +298,7 @@
 	var archConfig []archConfig
 	if Bool(config.Mega_device) {
 		archConfig = getMegaDeviceConfig()
-	} else if Bool(config.Ndk_abis) {
+	} else if config.NdkAbis() {
 		archConfig = getNdkAbisConfig()
 	}
 
@@ -862,6 +861,10 @@
 	return ok
 }
 
+func (c *config) NdkAbis() bool {
+	return Bool(c.productVariables.Ndk_abis)
+}
+
 func stringSlice(s *[]string) []string {
 	if s != nil {
 		return *s