aconfig: move `cache_` as a memeber

provider_ object relies on cache_ vector to be alive, but the order
of destruction between these objects at exit are not guaranteed.
This can lead to crash at the exit especially on ASAN build.
By putting cache_ as a member it is guaranteed that cache_ is not
destructed until provider_ is destructed.

Test: m ; m AconfigDemoActivity
Test: atest aconfig.test
Test: observe no crash on _hwasan build
Change-Id: If9e23e99c501bf8f06fcab003622948b9e730352