Improve GATT Server database handling.

- use references instead of pointers where possible. Thanks to it we can
  remove some reduntant null checks and simplify the code.
- use directly allocated memory instead of own buffers. Thanks to it the
  stack uses around 12kb less of memory after startup.
- use list and vector from std library, instead of some hand-written
  implementations.

This patch is a prerequisite for further server refactoring, that will
further reduce the space used, make unit-testing possible, and enable
proper handling of PTS GATT tests.

Test: sl4a GattReadTest
Bug: 38225928
Change-Id: I1620be682259ccb8f0c02754806e355e3f1ad0c1