Enable overrides and add stem library properties

This allows modules to override exiting libraries
with custom implementations.

Bug: http://b/114470065
Test: make
Change-Id: I2692a5416a2620ab2268df8998dd2620074adc1f
diff --git a/cc/androidmk.go b/cc/androidmk.go
index 69ed771..5e38973 100644
--- a/cc/androidmk.go
+++ b/cc/androidmk.go
@@ -143,6 +143,9 @@
 		ret.Extra = append(ret.Extra, func(w io.Writer, outputFile android.Path) {
 			fmt.Fprintln(w, "LOCAL_SOONG_TOC :=", library.toc().String())
 			fmt.Fprintln(w, "LOCAL_SOONG_UNSTRIPPED_BINARY :=", library.unstrippedOutputFile.String())
+			if len(library.Properties.Overrides) > 0 {
+				fmt.Fprintln(w, "LOCAL_OVERRIDES_MODULES := "+strings.Join(library.Properties.Overrides, " "))
+			}
 		})
 	} else if library.header() {
 		ret.Class = "HEADER_LIBRARIES"