libprocessgroup: Explicit field presence for MaxActivationDepth

https://protobuf.dev/programming-guides/field_presence/

Explicit field presence is required to support a MaxActivationDepth
value of 0 (the implicit default) for the libprocessgroup_proto_test.
Other non-zero values work fine.

All (positive) values of MaxActivationDepth work correctly at runtime.
This is a test-only fix.

Test: atest libprocessgroup_proto_test
Bug: 346584259
Bug: 377579705
Fixes: 28b37f27 ("libprocessgroup: Add MaxActivationDepth")
Change-Id: Ic3acc85790e1c84bef3f2bb50399011f28339c3b