Extract duplicate code to common helper functions.

Also, fall back to using a default name for the dexpreopt directory if
we are not building for Android.

Bug: 290583827
Test: m nothing
Change-Id: I3fc6ff9142a2dcdf995796f75891b242fe2848d0

java/hiddenapi_singleton.go

@@ -166,7 +166,7 @@ func isModuleInConfiguredList(ctx android.BaseModuleContext, module android.Modu
 
 	// Now match the apex part of the boot image configuration.
 	requiredApex := configuredBootJars.Apex(index)
-	if requiredApex == "platform" || requiredApex == "system_ext" {
+	if android.IsConfiguredJarForPlatform(requiredApex) {
 		if len(apexInfo.InApexVariants) != 0 {
 			// A platform variant is required but this is for an apex so ignore it.
 			return false