Apex: support codenames for min_sdk_version

Apex can use codenames like "Q", "R" for its min_sdk_version property.
Also, cc_library can use codenames for its stubs.versions.

Bug: 152655956
Test: vendor/google/build/build_mainline_modules.sh
Change-Id: I077ad7b2ac5d90b4c8708921e43846206f05ba70
diff --git a/cc/ndk_library.go b/cc/ndk_library.go
index 00338b9..eb8e9d3 100644
--- a/cc/ndk_library.go
+++ b/cc/ndk_library.go
@@ -96,6 +96,8 @@
 	Unversioned_until *string
 
 	// Private property for use by the mutator that splits per-API level.
+	// can be one of <number:sdk_version> or <codename> or "current"
+	// passed to "gen_stub_libs.py" as it is
 	ApiLevel string `blueprint:"mutated"`
 
 	// True if this API is not yet ready to be shipped in the NDK. It will be