Clear reliable message fields for public input NanoAppMessage

This CL clears the isReliable and messageSequenceNumber fields
in the NanoAppMessage class for inputs to public APIs. These
fields are used internally to track messages, and the public
setter is needed for this purpose.

WANT_LGTM=lelandr

Bug: 328120618
Change-Id: I3fb7a2893e77c7a551c479a4ccee5402d6e5bc78
Test: Presubmits