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