aconfig: rename subcommand 'dump' -> 'dump-cache'

Rename the 'dump' subcommand to 'dump-cache' for better symmetry with
'create-cache', and to prepare for a potentially upcoming
'dump-storage' subcommand.

Keep 'dump' as an alias for 'dump-cache' for backwards compatibility.

Bug: N/A
Test: m
Change-Id: I521ce507985e4cf450ee1b8517993b798f11430c