Rename sdkMemberRef to sdkMemberVariantDep

Previously, there were two types sdkMemberRef and sdkMember that seemed
as though the former was referring to the latter but that was not the
case. The former was actually information about the dependency the sdk
had on a specific member variant. The latter is all the information
about a specific member, including all its variants.

This change attempts to clarify that relationship, both through a
better type name and through some improved documentation.

Bug: 186290299
Test: m nothing
Change-Id: I1dcc0ddf3333da9e797bb23ae945d04729839014
diff --git a/sdk/sdk.go b/sdk/sdk.go
index b60fb18..710c1f2 100644
--- a/sdk/sdk.go
+++ b/sdk/sdk.go
@@ -54,14 +54,14 @@
 	// list properties, e.g. java_libs.
 	dynamicMemberTypeListProperties interface{}
 
-	// Information about the OsType specific member variants associated with this variant.
+	// Information about the OsType specific member variants depended upon by this variant.
 	//
 	// Set by OsType specific variants in the collectMembers() method and used by the
 	// CommonOS variant when building the snapshot. That work is all done on separate
 	// calls to the sdk.GenerateAndroidBuildActions method which is guaranteed to be
 	// called for the OsType specific variants before the CommonOS variant (because
 	// the latter depends on the former).
-	memberRefs []sdkMemberRef
+	memberVariantDeps []sdkMemberVariantDep
 
 	// The multilib variants that are used by this sdk variant.
 	multilibUsages multilibUsage