AssembleVintf: fix error message for unsupported tags/attrs for additional manifests

The error message used to contain extraneous values like:

<manifest version="1.0" type="device" target-level="1">
   <sepolicy>
       <version>0.0</version>
   </sepolicy>
</manifest>

Fix it so that it becomes:
<manifest target-level="1" />

Test: manual with additional manifest, then
      `m device_manifest.xml`

Bug: 78943004

Change-Id: I2d66c4db5a5c2136907ac2dc814262909c73b9e0
4 files changed