Make pom2bp output preprocessed: true on app imports

The only difference between preprocessed + presigned vs just presigned
is that just presigned will zip align the apk, and preprocessed +
presigned will instead check that the apk is already aligned and has
uncompressed jni and dex libs. This should be the default to ensure
we're getting valid apk imports from external sources.

Presigned alone also won't work for apps with target sdk >= 30, because
on those target sdks you have to use apk signature v2, and having the
build system align the zip file will break that signature.

Bug: 185811447
Test: Presubmits
Change-Id: Ie825ab2eab4feeda5c266233a9dca7bbf9559be7

cmd/pom2bp/pom2bp.go

@@ -556,7 +556,8 @@ var bpTemplate = template.Must(template.New("bp").Parse(`
     {{- end}}
     {{- end}}
     {{- if .IsApk}}
-    presigned: true
+    preprocessed: true,
+    presigned: true,
     {{- end}}
 
 }