dlog_logger: lazy poll only for Logger backend

dlog_logger: lazy poll only for Logger backend

The delay makes sense for Android Logger because it only reads logs
from an "external" (from the daemon's PoV) storage so they aren't
going anywhere, and it's good not to waste resources trying to read
them at boot time.

Meanwhile for the Pipe backend it is important for the daemon to
actively listen to clients who might be trying to write logs, since
in that case the daemon is the storage (so logs need to be written
immediately, for example for crash report reasons) and also the
implementation requires it (since pipes otherwise get clogged).

Change-Id: I215438459c334698b4f3c195f6fce69ab7454f9c
Signed-off-by: Michal Bloch <m.bloch@samsung.com>