Support cpp backend for libfmq

Before this CL, libfmq only supports the NDK backend for AIDL.
Add support for the cpp backend as well.

Bug: 362791857

Test: atest fmq_test

Flag: EXEMPT; Header changes, no runtime flags.

Change-Id: I0654ef71b83236afd9614a14eb5af359dac16420
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>