When report_on_target option is set, binary trace will be "reported"
into a text version on the target device. This removes the need for
trace-cmd to be installed on the host, in which case that should not be
reported as an error.
The warning refers to trace.bin, which is not the extension that we use
for it. Instead, use the variable with the default trace
name (trace.dat) for the warning.
The default value for buffer_size_file contained a path under "/d/",
which is an Android-specific alias for "/sys/kernel/debug". This commit
updates the default value to use the system-agnostic path.
