ContextHubService: Start message sequence number randomly

This CL reduces the probability of the following
scenario:

1. A host endpoint sends a reliable message.
2. The service restarts.
3. A host endpoint sends a new reliable message with the
same message sequence number (always start with 0).
4. CHRE sends a response to the
message from (1).
5. The service completes the message in (3) incorrectly.

Bug: 333567700
Change-Id: Iecdb9e7492cb8ead71b8b3580c05018a4df4e463
Test: Presubmits