Fix a debug-output formatting error

The function that prints the list of pids watched by the ANR tracing
feature is supposed to inject a comma between pids.  A variable
assignment was missing in the original code.

Tested with the following command:
`adb shell am trace-timer pid=2000,2001 expire=20`

Bug: 352085328
Flag: com.android.server.utils.anr_timer_trace
Test: atest
 * FrameworksServicesTests:com.android.server.am
 * FrameworksServicesTests:AnrTimerTest
Change-Id: Ib3a70bc27299ad2458cd6a3936e2b6632d6dd884