Remove the one receiver limit for a broadcast to be considered "urgent".

The goal of this "urgent" concept is to make sure certain broadcasts
such as those sent with FLAG_RECEIVER_FOREGROUND are not blocked behind
normal broadcasts and the current API contract for FLAG_RECEIVER_FOREGROUND
doesn't specify any limit on no. of receivers.

Bug: 263803618
Test: atest services/tests/mockingservicestests/src/com/android/server/am/BroadcastQueueTest.java
Test: atest services/tests/mockingservicestests/src/com/android/server/am/BroadcastQueueModernImplTest.java
Change-Id: I5b6b851aa5b735f0290b8fc4870468c8627b3aba