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/cc/ndk_headers.go b/cc/ndk_headers.go
index cdf63d8..a7d6e04 100644
--- a/cc/ndk_headers.go
+++ b/cc/ndk_headers.go
@@ -134,7 +134,7 @@
 	// but keep them when doing regular platform build.
 	// Ndk_abis property is only set to true with build/soong/scripts/build-ndk-prebuilts.sh
 	// TODO: Revert this once MIPS is supported in NDK again.
-	if Bool(ctx.AConfig().Ndk_abis) && strings.Contains(ctx.ModuleName(), "mips") {
+	if ctx.Config().NdkAbis() && strings.Contains(ctx.ModuleName(), "mips") {
 		return
 	}