Fix potential overflow of launch event uptime

It was stored as integer for legacy MetricsLogger to log in seconds.
But since atom log uses it in milliseconds, it may be overflow if the
time is greater than 24 days. So now just store it in milliseconds
with long type and only covert to seconds for legacy logger.

Bug: 191610816
Test: atest ActivityMetricsLoggerTests

Change-Id: I3e40097fe7ce8e01d8495c1de2f1eafa855934a8