adb: tracing: don't make strings if runtime tracing is disabled

If tracing was not enabled (the ADB_TRACE environment variable was not
set specially), writex() and readx() would still call dump_hex() which
would construct hex tracing strings, which would be immediately
discarded and not printed (because tracing is not enabled).

The fix is to only call dump_hex() if ADB_TRACING evalutes to true, the
same way that dump_packet() is only called if ADB_TRACING evaluates to
true.

Change-Id: I1651680da344389475ebdeea77ba1982960d5764
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>