Generate api_levels.json from the preview codenames.

When the branch is configured as something less than the max codename
(for example, right now AOSP is U but V is also in development), the
active codenames list will not include V.

Bug: None
Test: None
Change-Id: Ia22388a8ba94ff00d053acb33363c3cdce7677d0

android/config.go

@@ -923,6 +923,11 @@ func (c *config) PlatformVersionActiveCodenames() []string {
 	return c.productVariables.Platform_version_active_codenames
 }
 
+// All unreleased codenames.
+func (c *config) PlatformVersionAllPreviewCodenames() []string {
+	return c.productVariables.Platform_version_all_preview_codenames
+}
+
 func (c *config) ProductAAPTConfig() []string {
 	return c.productVariables.AAPTConfig
 }