Have bpfmt show usage when run without any arguments

This is partially in hopes of making it generally easier to use, and
partially in hopes of specifically making it less tempting to do

bpfmt Android.bp > Android.bp

which actually would just erase the contents of Android.bp

Test: bpfmt Android.bp 2>&1 | grep usage && echo ok
Bug: 67326589
Change-Id: I41c23aca6214f27dbdf55e5931f9392c318e82ed