Change type of Unbundled_build_apps from bool to []string

Unbundled_build_apps is supposed to be a list of apps to
build as unbundled, not just a simple boolean. Change it
to properly reflect what it is, because the starlark
product configuration relies on soong's interpretation
of variables types.

Bug: 201700692
Test: Presubmits
Change-Id: I85d40d58fe519f9f6e5a382b8f3ec9014f990aa1
diff --git a/android/config.go b/android/config.go
index 5ee28e7..76bfc28 100644
--- a/android/config.go
+++ b/android/config.go
@@ -856,7 +856,7 @@
 // Returns true if building apps that aren't bundled with the platform.
 // UnbundledBuild() is always true when this is true.
 func (c *config) UnbundledBuildApps() bool {
-	return Bool(c.productVariables.Unbundled_build_apps)
+	return len(c.productVariables.Unbundled_build_apps) > 0
 }
 
 // Returns true if building image that aren't bundled with the platform.