Add debug ramdisk variant.

A module will be installed to /debug_ramdisk if debug_ramdisk is set to
true.

This is a reland of f84e9c05e2, with a fix
that removes /first_stage_ramdisk.

Bug: 184004542
Test: soong test
Change-Id: I739de63cfec6b0fec5a90f7c4741fc4d884d209c

rust/image.go

@@ -97,6 +97,10 @@ func (mod *Module) RamdiskVariantNeeded(android.BaseModuleContext) bool {
 	return mod.InRamdisk()
 }
 
+func (mod *Module) DebugRamdiskVariantNeeded(ctx android.BaseModuleContext) bool {
+	return false
+}
+
 func (mod *Module) RecoveryVariantNeeded(android.BaseModuleContext) bool {
 	return mod.InRecovery()
 }