Define EventQueueFlagBits

The EventQueueFlagBits is used to notify the other end of an FMQ when
an action should be taken. For instance, the read end of the queue is
able to wait on a particular mask bit to be set by the write end of
the queue before reading events.

This is necessary because the number of events to read must be
specified when calling the FMQ's read() or readBlocking() functions.
read() will fail if the specified number of events is not available.
readBlocking() will not perform a partial read, so if the number of
events to read is not known ahead of time, the function may block for
an extended period of time until enough events are queued up. By
utilizing the EventFlag, the reading of events is able to wait until
events are ready to be read, and then the availableToRead() function
provides the number of events to read when calling read() or
readBlocking().

Bug: 111070257
Test: Compiles
Change-Id: I20e9e566efc58196e27e50dae3fc13518ce605fc