dumpsys: add --thread option

This displays the thread usage of services in the format
"<threadsUsed>/<threadCount>". This has been useful in lshal to find
deadlocks in binder threads.

Test: atest dumpsys_test
Test: run 'dumpsys --thread' on cuttlefish and verify thread usage
Test: run 'dumpsys --thread SurfaceFlinger' and verify thread usage
Bug: 140639610
Change-Id: I0cd3f86ca47ec911b276d8e8859ef54743ee34cf