floss: Output dumpsys to a fixed temp file

dumpsys command sometimes encounters D-Bus timeout error.

This patch mitigates the timeout by writing diagnostic information
to a fixed temporary file and returning the file path instead of
the content, thereby reducing IO time.

Bug: 327542410
Tag: #floss
Test: btclient -c dumpsys
Test: m com.android.btservices
Flag: EXEMPT, floss only change

Change-Id: I581ed794d5464531944435b7b34ed38017354503