build: Set QCOM variants for non-QCOM_HARDWARE defined targets

* Nexus devices and others typically do not define the QCOM_HARDWARE
  flag, so the variant path should always default to the AOSP variant.
* Unconditionally set the variant to the AOSP HAL by default.

Change-Id: I714170897128f92718af266366cfcbf3136e8981