pmqos: Fix to return proper value for dbus signal handler

pmqos: Fix to return proper value for dbus signal handler

From the dbus-stub generated code, signal handler should return
TRUE if it handles invocation. Even ret value is error, fix to
return TRUE after handling invocation.

Below is dbus-stub comment about signal handler:
  If a signal handler returns %TRUE, it means the signal handler
  will handle the invocation and no other signal handlers will
  run. If no signal handler handles the invocation, the
  %G_DBUS_ERROR_UNKNOWN_METHOD error is returned.
  Returns: %G_DBUS_METHOD_INVOCATION_HANDLED or %TRUE if the
		   invocation was handled,
		   %G_DBUS_METHOD_INVOCATION_UNHANDLED or %FALSE to let
		   other signal handlers run.

NOTE: GLIB related asan heap-buffer-overflow issue is from the
wrong return.

Change-Id: I070c4a3fd12d5421cc52e69e6ada25b2ec772e6f
Suggested-by: Sangjung Woo <sangjung.woo@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>