Add TARGET_USES_AOSP_FOR_WLAN soong variable

It's set and used in many .mk files. Adding this to "wifi" soong config
namespace helps migration of those .mk files into .bp files.

Bug: 273376293
Test: m libwifi-hal libwifi-hal-qcom
Merged-In: Ie0fad26305d50f70076b7e538fd1a7fa03e397cb
Change-Id: Ie0fad26305d50f70076b7e538fd1a7fa03e397cb