vts: Use global hostapd_ since teardown will call terminate function

aosp/1427815 add invoke terminate function in teardown function.
It will cause Create Test case fail since Create killed global hostapd_
service to re-generate the local one.
Use global hostapd_ to replace local variable to make teardown function
work normally.

Bug: 169248773
Test: atest -c VtsHalWifiHostapdV1_2TargetTest
Test: atest -c VtsHalWifiHostapdV1_1TargetTest
Test: atest -c VtsHalWifiHostapdV1_0TargetTest
Change-Id: I6300a626db76e68873bbc34c489a9a8ee2d55e7a