Don't use prebuilt_apex when TARGET_FLATTEN_APEX is set to true

When the device does not support non-flattened APEX (i.e.
TARGET_FLATTEN_APEX is set to true), then ignore the prebuilt_apex
modules even when `prefer: true`.

Bug: 136251130
Bug: 136662772
Test: build marlin, and check if /system/apex has flattened APEXes
Change-Id: I9f3dfefc3bd357d6750fbb1e418937d095720b04
diff --git a/android/prebuilt.go b/android/prebuilt.go
index 2556770..3be10f7 100644
--- a/android/prebuilt.go
+++ b/android/prebuilt.go
@@ -178,3 +178,7 @@
 
 	return source == nil || !source.Enabled()
 }
+
+func (p *Prebuilt) SourceExists() bool {
+	return p.properties.SourceExists
+}