wlan: update project discrimination in Android.mk

The prima and qcacld-2.0 projects both use the WLAN_CHIPSET variable
to keep track of whether or not the given project is supported on a
given platform.  The problem is that both of these assume WLAN_CHIPSET
is initially empty, but that is not always the case.  When both are
included in an Android build, the value may be set by one of the
projects, and that value will be inherited by the other project.  To
prevent this explicitly clear the WLAN_CHIPSET variable in both
projects.

Change-Id: I138efff35a1d82e5e13e43b603df350b092d4fb5