Handle the 'enabled' property in bp2build

Also fix some bugs pertaining to configurable attribute handling of bool
attributes and label sttributes, so that they may support values across
multiple different axes at the same time.

Test: unit tests for bp2build
Test: mixed_droid

Change-Id: I411efcfddf02d55dbc0775962068a11348a8bb2c

etc/prebuilt_etc.go

@@ -681,7 +681,8 @@ func prebuiltEtcBp2BuildInternal(ctx android.TopDownMutatorContext, module *Preb
 				continue
 			}
 			if props.Src != nil {
-				srcLabelAttribute.SetSelectValue(axis, config, android.BazelLabelForModuleSrcSingle(ctx, *props.Src))
+				label := android.BazelLabelForModuleSrcSingle(ctx, *props.Src)
+				srcLabelAttribute.SetSelectValue(axis, config, label)
 			}
 		}
 	}