[Pandora] Add GATT server tests

Added GATT server tests + some fixes for advertising and
first pass on GATT server.

Tag: #feature
Bug: 245578454
Test: Test: atest pts-bot:GATT/SR
Ignore-AOSP-First: Cherry-pick from AOSP
Merged-In: I2482a9a7f91969778c69503d1e14bf6c4221ac0d
Change-Id: I2482a9a7f91969778c69503d1e14bf6c4221ac0d