Use proptools.BoolDefault

Replace x == nil || *x with proptools.BoolDefault(x, true)

Test: m checkbuild
Change-Id: Icc5378cab3ea72d86544b40761c2f00b278cc0a1
diff --git a/java/androidmk.go b/java/androidmk.go
index fff034c..c9a05c3 100644
--- a/java/androidmk.go
+++ b/java/androidmk.go
@@ -235,7 +235,7 @@
 		Include:    "$(BUILD_SYSTEM)/soong_java_prebuilt.mk",
 		Extra: []android.AndroidMkExtraFunc{
 			func(w io.Writer, outputFile android.Path) {
-				if jd.properties.Installable == nil || *jd.properties.Installable == true {
+				if BoolDefault(jd.properties.Installable, true) {
 					fmt.Fprintln(w, "LOCAL_DROIDDOC_DOC_ZIP := ", jd.docZip.String())
 				}
 				if jd.stubsSrcJar != nil {
@@ -253,7 +253,7 @@
 		Include:    "$(BUILD_SYSTEM)/soong_java_prebuilt.mk",
 		Extra: []android.AndroidMkExtraFunc{
 			func(w io.Writer, outputFile android.Path) {
-				if ddoc.Javadoc.properties.Installable == nil || *ddoc.Javadoc.properties.Installable == true {
+				if BoolDefault(ddoc.Javadoc.properties.Installable, true) {
 					fmt.Fprintln(w, "LOCAL_DROIDDOC_DOC_ZIP := ", ddoc.Javadoc.docZip.String())
 				}
 				if ddoc.Javadoc.stubsSrcJar != nil {