GATT cache memory corruption fix

When doing GATT discovery, we interleave adding elements to vector,
and storing pointers to them. This can cause memory corruption, because
vector can reallocate the memory when adding element, leaving the pointers
invalid. To fix that, never store pointers to mutable vector elements.

Bug: 70041392
Test: manual, connected to HID device
Change-Id: I6bb3fec61afa742f446b66e3291c5f326738ee17