Floss: Pandora GATT: Implement GATT server

-Handle characteristic/descriptor read requests.
-Handle characteristic/descriptor write requests.
-Define required services, characteristics and descriptors for PTS.

Bug: 300954033
Test: mma packages/modules/Bluetooth && pts-bot GATT
Tag: #floss
Flag: EXEMPT floss only changes
Change-Id: I3f67b3a5b50090eb7704a74157fa8f3697820033