soong: Align update boot jar variable with system_server jar

Use apex:jar pair values for updatable boot jars to align with
updatable system_server values.

Test: Compiles & flashed device. Ensured that the bootclasspath & system_server class
paths remain the same.

Change-Id: I1d6dfe3325d9091b7c724458be4e6471f9086666

dexpreopt/dexpreopt.go

@@ -554,6 +554,12 @@ func SplitApexJarPair(apexJarValue string) (string, string) {
 	return apexJarPair[0], apexJarPair[1]
 }
 
+// Expected format for apexJarValue = <apex name>:<jar name>
+func GetJarLocationFromApexJarPair(apexJarValue string) (string) {
+	apex, jar := SplitApexJarPair(apexJarValue)
+	return filepath.Join("/apex", apex, "javalib", jar + ".jar")
+}
+
 func contains(l []string, s string) bool {
 	for _, e := range l {
 		if e == s {