Add debug ramdisk variant

A module will be installed to debug_ramdisk (or
debug_ramdisk/first_stage_ramdisk if recovery as boot is true) if
debug_ramdisk is set to true.

Bug: 184004542
Test: soong test
Change-Id: Ic5a4d27407e506fffa462de2149e0785f11b2ac7

cc/snapshot_prebuilt.go

@@ -308,6 +308,10 @@ func (s *snapshot) VendorRamdiskVariantNeeded(ctx android.BaseModuleContext) boo
 	return false
 }
 
+func (s *snapshot) DebugRamdiskVariantNeeded(ctx android.BaseModuleContext) bool {
+	return false
+}
+
 func (s *snapshot) RecoveryVariantNeeded(ctx android.BaseModuleContext) bool {
 	return false
 }