Fixed null callback issue

Fixed the vendor data/network service non-responsive
issue. The callback binder passed from frameworks
might be GC'd so that vendor data/network service skipped
calling the callback. This eventually caused data
connection state machine messed up.

Fixed by turning the weak refernce to the callback into
a strong reference. This ensure the binder alive when
vendor service needs to invoke the callback.

Test: Telephony sanity tests
Bug: 139076980
Change-Id: Ica0b7b810ffd5416ffd1b2b61f7ebc4af0dcb8ce