Floss: Fork Floss D-Bus gatt_client from autotest_lib.

Changes made afterwards:
*Lint the forked code as Google Python style.

Bug: 307324838
Test: mma packages/modules/Bluetooth
Tag: #floss
Flag: EXEMPT floss only changes
Change-Id: Icc5d86fc04cfde44ff9f085034085950ac26662d