WM: Don't empty Winscope traces when writing to file.

WM empties its trace buffer when writing it to file. It should instead
keep the buffer in case the data is necessary in another bug report.

Test: Flash a device. Start WM trace. Use the device. Dump WM trace.
Check WM trace status (it should not be empty)

Change-Id: Ic54bbfed4b5930496de7f081d96f3ae67b2d2f69