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

sdk/cc_sdk_test.go

@@ -2126,6 +2126,10 @@ myinclude/Test.h -> include/myinclude/Test.h
 		)
 	}
 
+	t.Run("ramdisk", func(t *testing.T) {
+		testImageVariant(t, "ramdisk_available", "ramdisk_image_required")
+	})
+
 	t.Run("recovery", func(t *testing.T) {
 		testImageVariant(t, "recovery_available", "recovery_image_required")
 	})