libbinder: add IBinder::withLock

BpBinder (and BBinder, once its mExtra allocation is made) have very
nice and shiny locks which they keep all for themselves! Stop it,
IBinder! Share that lock!

This provides convenient access to IBinder's lock, in order to avoid
needing additional locks elsewhere.

Bug: 192023359
Test: N/A
Change-Id: Id3485a2ac66d19379dcad2f0b41d6cb7a8a96725