permission_broker: Fixes to run with gtest-1.6

* Programatically calculate gmock/gtest library dependencies.
* Mock out WaitForEmptyUdevQueue() while running test. The unit test
  hangs at this call with gtest 1.6. We shouldn't be using the real
  implementation while running tests anyway.
* Make PermissionBroker destructor virtual.

TEST=built and ran permission_broker unit tests against gmock/gtest-1.6

Change-Id: I9eef56acbe7956b07f77b1a1e98b1cdf72af0460
3 files changed