Fix tests/runtests.sh

The location of FrameworksWifiTests.apk installed by the script
with adb seems to have changed. The path of the APK contains the
TARGET_ARCH environment variable, which is now fetched
with a similar method as in system/netd/tests/runtests.sh.

Noticed the issue with "lunch aosp_x86_64-eng". Before this fix,
the test script produced an error:

    adb: failed to stat [...]/FrameworksWifiApiTests.apk:
    No such file or directory

Bug: 128603658
Test: ./frameworks/base/wifi/tests/runtests.sh

Change-Id: Id4a33ca89fa21a9a4ef791c5c1c5760e0e2badaf
Signed-off-by: Otto Seiskari <otto.seiskari@indooratlas.com>