Force dumpvars into dumb terminal mode and move log files

Force soong_ui in dumpvars mode to use a dumb terminal so that it
doesn't print an empty status table.  Also move the logs to files
that are prefixed with "dumpvars-" so that they don't rotate the
real build logs.

Fixes: 139307849
Test: lunch
Change-Id: If92421af07731fedc9ca42670f309e95922a2224