binder: SharedRefBase uses enable_shared_from_this

So that make_shared of a SharedRefBase object will not cause
double-ownership. For code compat reasons, we can't actually remove this
class (many places reference it).

Fixes: 194905504
Test: CtsNdkBinderTestCases
Change-Id: I04671965d97960d6630004883df77c17de066d4b