Use alias for shared_ptr<FontFamily>.

On aarch64, libc++'s shared_ptr allocates 24 byte [1] or 32 byte [2]
reference counting object per one referenced object.

We can save memory by creating a shared_ptr<vector<FontFamily>> and
reusing its reference counting object.
We can use aliasing constructor to share the reference counting object
between shared_ptr<vector<FontFamily>> and shared_ptr<FontFamily>.

This saves about ~6 KB (32 bytes * 198) on oriole.

[1] 24 bytes (__shared_ptr_emplace) for make_shared<T>()
[2] 32 bytes (__shared_ptr_pointer) for shared_ptr<T>(new T)

Bug: 174672300
Test: atest minikin_tests
Change-Id: Ife21c1f13580273535847fc9351bd5def71c4a10