Add ramdisk image sdk trait to cc_library_headers

Allows an sdk to require that a cc_library_headers module provides a
ramdisk image variant for the prebuilt.

Previously, "ramdisk_available: true" would be set in the generated
prebuilt snapshot for any sdk member that specified
"ramdisk_available: true" in the source module. This change will only
add that setting to the snapshot if the ramdisk image variant trait
was explicitly requested for a member.

Bug: 195754365
Test: m nothing
Change-Id: I21e0dd02d380beabf228a72c463fcc603c77d87f
diff --git a/cc/image_sdk_traits.go b/cc/image_sdk_traits.go
index 371bb24..1d28230 100644
--- a/cc/image_sdk_traits.go
+++ b/cc/image_sdk_traits.go
@@ -19,6 +19,7 @@
 // This file contains support for the image variant sdk traits.
 
 func init() {
+	android.RegisterSdkMemberTrait(ramdiskImageRequiredSdkTrait)
 	android.RegisterSdkMemberTrait(recoveryImageRequiredSdkTrait)
 }
 
@@ -26,6 +27,12 @@
 	android.SdkMemberTraitBase
 }
 
+var ramdiskImageRequiredSdkTrait android.SdkMemberTrait = &imageSdkTraitStruct{
+	SdkMemberTraitBase: android.SdkMemberTraitBase{
+		PropertyName: "ramdisk_image_required",
+	},
+}
+
 var recoveryImageRequiredSdkTrait android.SdkMemberTrait = &imageSdkTraitStruct{
 	SdkMemberTraitBase: android.SdkMemberTraitBase{
 		PropertyName: "recovery_image_required",