btif: Fix possible races on bluetooth off

Normally it is Java layer having access to btif interface, where calles
are binded to the blueototh mainthread.
However, some calls e.g. RemoveDevice are called from the bluetooth mainthread.
This happens when device got unbonded.
If we are unluckly, if during mainthread operation, there is Cleanup
scheduled due to Bluetooth being turning OFF, it might happen that
RemoveDevice operation will be scheduled, but when it start to execute,
services instances are already gone. This will lead to crash.

This patch add simple mechism to prevent that.

Bug: 226276953
Test: manual testing
Tag: #feature
Change-Id: Ib226e851981dadcc5b0091a1ac1428adcb7302a2