Don't send idle-maint trigger broadcast as ordered

Using an ordered broadcast has been inadvertently making the idle
trigger wait in line behind all other pending broadcasts.  Don't do
that -- it's going to registered receivers only anyway.

We now see no measurable dispatch latency for the trigger broadcast
even if it's issued while the BOOT_COMPLETED broadcast is still
being processed.

Bug: 77230503
Test: adb shell cmd activity idle-maintenance
Test: manually set idle threshold to 10s and observe triggering
Change-Id: I75a9f9acf0debfd075750d87c65cda545af48299