Only write missed AL logs on the first service start

Only write missed AL logs on the first service start

The AL backend is based on the kernel devices, which may be written to
at any time. dlog_logger used to read the whole contents of the device
on its start, which is the reasonable thing to do (we don't want to miss
any logs) but bad for various reasons (for example, the same logs are
written to the persistent logs file many times when the service is
restarted). This patch makes it so that the device is only read in its
entirety on the first start of the service after reboot; on every other
start, all the logs existing in the device at the moment of the service
start are skipped.

Change-Id: I1eefd62191d66201ca74919d32b27aecbb1f621b