Add another constructor to MQDescriptor.

The new constructor for MQDescriptor will create
a default GrantorDescriptor with fdIndex for all
grantors set to 0 and contiguous offsets.

Bug: 31710802
Test: Built and run all existing FMQ unit tests
and benchmarks.

Change-Id: Ic4ad707917bf37be2a44c65c1b101ee2bd1e2290
2 files changed