Add BufferClient information to dumpsys

Now "adb shell dumpsys bufferhubd" will tell you how many buffers have
been allocated via the binder service as well.

The reason for not having buffer info is because the info should be
stored with BufferNode, which is yet another pending CL. After
b/117790952 is resolved we can print them.

Test: run "adb shell dumpsys bufferhubd" after "atest
buffer_hub_binder_service-test" should print "Active BufferClients: 1"
Bug: 116526156

Change-Id: I72df3a2d4795bdba0ddf42c1a27a74cd5987f3d8