FTL: Allow implicit conversions with NonNull<T>

C++ allows certain implicit conversions for pointers, such as the
implicit conversion of "T*" to "const T*".

NonNull did not allow them, but that is easily corrected.

Bug: 185536303
Flag: EXEMPT Relaxes a compile-time constraint
Test: atest ftl_test
Change-Id: I0c14805c8fdcca979c67d2c86d57e5e678aa1b32