Cert: Use site-packages instead of root

* Symbolic link libraries in site-packages instead of root one
* Now we can finally run cert tests in virtualenv without PYTHONPATH

Example command without PYTHONPATH:

    act.py -c $ANDROID_BUILD_TOP/system/bt/gd/cert/host_only_config.json -tf $ANDROID_BUILD_TOP/system/bt/gd/cert/cert_testcases_facade_only -tp $ANDROID_BUILD_TOP/system/bt/gd

Bug: 148147248
Test: set up virtual env and run tests
Change-Id: I42ba29602addab41059916071c5d0b42cf17fd8f