GATT cache storage refactoring

Currently when storing GATT cache into file, we save it piece
after piece, max 10 elements at once. This is not necessary,
we have enough space to just save all at once.
Also the logic doesn't need to be so complicated - no need
to state machine for such simple task.

Bug: 27455533
Change-Id: I82923dbedfb317835e45e13e0ea28151eb524d37