Add getStrongRefCountForNodeByHandle to ProcessState

ServiceManager can use this function to query individual services for their ref counts.

Bug: 143108344
Test: Manual (printed ref counts from servicemanager and confirmed that they matched the data in /d/binder/state)
Change-Id: I8c84f4b10b1c2b7690b39d1c4f4f50abd7d715b0