Adapt to PLATFORM_VERSION_FUTURE_CODENAMES.

Don't preserve the confusing names from make. Rename AllCodenames to
ActiveCodenames, add FutureCodenames, and add CombinedCodenames to
fetch the *real* AllCodenames.

To allow `introduced=P` in NDK library definitions, we need to know
that P exists. Use the combined list of current and future API
codenames generating stubs.

Test: make ndk
Test: check out/soong/api_levels.json
Bug: None
Change-Id: I435f9ce7446236edc268a84e33474044a55a6302
diff --git a/android/api_levels.go b/android/api_levels.go
index cf1011b..370d553 100644
--- a/android/api_levels.go
+++ b/android/api_levels.go
@@ -68,7 +68,7 @@
 		"N":     24,
 		"N-MR1": 25,
 	}
-	for i, codename := range ctx.Config().(Config).PlatformVersionAllCodenames() {
+	for i, codename := range ctx.Config().(Config).PlatformVersionCombinedCodenames() {
 		apiLevelsMap[codename] = baseApiLevel + i
 	}