Allow building unsigned APEXs payload for testing.

Bug: 148447155
Test: built a testing apex with unsigned payload.
Change-Id: I95aa3f11ff1adc1421fcd7ed5a356ee531a0a818

apex/builder.go

@@ -484,6 +484,10 @@ func (a *apexBundle) buildUnflattenedApex(ctx android.ModuleContext) {
 			optFlags = append(optFlags, "--no_hashtree")
 		}
 
+		if a.testOnlyShouldSkipPayloadSign() {
+			optFlags = append(optFlags, "--unsigned_payload")
+		}
+
 		if a.properties.Apex_name != nil {
 			// If apex_name is set, apexer can skip checking if key name matches with apex name.
 			// Note that apex_manifest is also mended.