Print the output from RBE bootstrap shutdown

The output from bootstrap would contain the a one-line summary of RBE
download / upload stats.

Bug: b/183007125
Change-Id: I90082a8433504e40bbf92992c3c8d1d656c49429
Merged-In: I90082a8433504e40bbf92992c3c8d1d656c49429