IsoManager: Unify protection against usage when not started

Unify approach to API call protection against null pointer dereference.
Also fix some C++ lint errors.

Bug: 350894286
Test: atest net_test_btm_iso
Flag: EXEMPT; Non-functional change, covered with unit tests
Change-Id: Ie414885ae33cdac2d29ddbcdc8fa3549b706fcc8