ANDROID_UTILS_REF_BASE_DISABLE_IMPLICIT_CONSTRUCTION

In form, inspired by ANDROID_BASE_UNIQUE_FD_DISABLE_IMPLICIT_CONVERSION.

We get occasional bugs about sp double-ownership. When this flag is
enabled, we have:
- you must construct RefBase objects using sp<>::make
- you must construct wp<> objects by converting them to sp<>
- if you want to convert a raw pointer to an sp<> object (this is
  possible since the refcount is used internally, and is used commonly
  on this*), then you must use 'assertStrongRefExists' semantics which
  aborts if there is no strong ref held. That is, if a client uses
  std::make_shared and then calls a function which internally used to
  call `sp<T>(this)`, you would now call
  `sp<T>::assertStrongRefExists(this)`, and the double ownership
  problem would become a runtime error.

Bug: 184190315
Test: libutils_test
Change-Id: Ie18d3146420df1808e3733027070ec234dda4e9d