IsoManager: Improve callback calls

Handles the use-after-free issue the proper and recommended way using
weak pointers. The unit tests were also added to make sure no call on
the already released object is made.

Bug: 288297494
Tag: #feature
Test: atest --host net_test_btm_iso --no-bazel-mode

Change-Id: I69ab25c1c19875142360436c52a428499198a1c3