floss: Add and remove PID files in btadapterd

As btif_dm_hw_on happens after GD_SHIM_MODULE's start_up,
creating the PID file in the start_up would allow upper
applications use the bluetooth adapter before it's fully
initialized.
This patch fixes the issue by creating and removing the PID
file in btadapterd while receiving AdapterState callback
from BTIF.

Bug: 262646536
Tag: #floss
Test: Manual - restart chromebook / toggle bluetooth adapter

Change-Id: I67afe41441dfd6494eea353c033df84d16e0885c