Add static_asserts for the size of InputMessage

Inside InputTransport.cpp, we are reading and writing InputMessages.
This is done in the following way:

read
::recv(getFd(), msg, sizeof(InputMessage), MSG_DONTWAIT);

write
::send(getFd(), &cleanMsg, msgLength, MSG_DONTWAIT | MSG_NOSIGNAL);

We are sending a variable-length message across the socket, and
receiving a maximum of sizeof(InputMessage) when reading it.

In this CL, we are adding asserts on the _maximum_ length of the message
that we would send across the socket. Since we typically only have a few
pointers at most, while MAX_POINTERS=16, in reality the communication
between system_server and app will use much less data.

However, it's still useful to add these asserts to understand the
worst-case scenario of message transfer.

Bug: 167946763
Test: m StructLayout_test
Change-Id: I281ecea62b392dea56936d031ab9c4ee18add93f