Drop 'isValid' optional field from EventLog

I laughed when I saw it, but anyhow 'isValid' is deliberately not marked as
'required'; you should only set if if you are performing client-side validation
of data, which you're not.
Rename 'schema_rev' => 'scid'

For convenience, the documentation and code for EventLogging refer
to (schema_name, revision_ID) pairs as 'SCIDs'. It'd be nice (but
not required) if android-commons stuck to this convention, too.