Change type of btgatt_gatt_id_t.inst_id to 16-bit

ARC++ uses instance id also as handles, so the type needs to be 16 bit
instead of 8 bit.

Bug: 191129417
Tag: #stability
Test: Manual test - ARC++ on zork fitbit setup flow with Versa 3

Change-Id: I2cc8609f611c558dc5f53198836db29f4ac9a2ae