Bluetooth: Discovery Timer stability fixes

This change prevents running timers from being re-initialized
which causes system instablility. Also prevents starting a
discovery operation when already in progress.

CRs-fixed: 328673
Change-Id: Icda36a25fdcb40dab4f95f9cc39ca124b299e308
Signed-off-by: Brian Gix <bgix@codeaurora.org>