Add an overload of IntentFilter#countActions() to avoid NPE.

Since IntentFilter.mActions could be null, calling countActions()
could result in a NPE. Add an overload of that method that takes
into account the fact that mActions could be null.

Bug: 336385821
Test: manual
Flag: com.android.server.am.trace_receiver_registration
Change-Id: I73c4238f037f1a6931a610cc5d207f30a59392ba