Clear TARGET_PRODUCT and TARGET_BUILD_VARIANT when getting lunch menu

An old TARGET_PRODUCT value pointing to a removed product or an invalid
TARGET_BUILD_VARIANT value cause lunch to fail to read
COMMON_LUNCH_CHOICES, clear them before calling into the build with
get_build_var.

Test: TARGET_PRODUCT=foobar TARGET_BUILD_VARIANT=foobar lunch
Change-Id: I53d205784149ba7e32cc093f9d81d1a82c08bfc0