media_recorder: fix start time in startRecording

Whenever startRecording() is called and path_ is not empty (new API)
the start time variable is left uninitialized.

This commit addresses this issue.

Change-Id: I9020e91695c6803d881967999d868d4ba4fd1c72
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>