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

android/variable.go

@@ -239,6 +239,8 @@ type productVariables struct {
 	BoardPlatPrivateSepolicyDirs []string `json:",omitempty"`
 
 	VendorVars map[string]map[string]string `json:",omitempty"`
+
+	Ndk_abis *bool `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {