BTM: Properly merge LE scan from BTIF and CSIS

If CSIS scan is added while BTIF is scanning, we would return error code
that shuts down both scans in BTA layer.
This code would instead keep both scans running. We would ingore the
duration CSIS passes, assuming whoever started the scan, will not shut
it down instantly.

Bug: 273958309
Test: connect to CSIS device while some app in Java is doing scan
Change-Id: Ibb22e2722d558be49523a9b159267114b8971d47