Fix WM dump perfetto slice

Move perfetto slice within WindowManagerService#dumpDebugLocked().

The slice was previously traced within WindowTracing and other entry
points (e.g. WindowManagerService#doDump()) were not triggering it.

Flag: EXEMPT minor fix
Bug: 369592408
Test: presubmit
Change-Id: I9019dc64392f9c4aa3e592ea07bda5b1297f8717