LeAudioTestApp: Fix passing BASS scan context

This cleans up broadcast sources scanning code in case when
there is no scan delegator device. It removes the confusing
code with fake device.

Scanning from the main activity gives no device context and it
does not support any Broadcast Scan Assistant specific actions
besides the scanning, which is the only action that requires no
remote device. Scanning on behalf of a scan delegator device is
suppose to be triggered from the LeAudioRecycleViewAdapter.
This is suppose to be the point where one interacts with remote
BASS device, adds, modify or removes broadcast sources.

Bug: 150670922
Tag: #feature
Test: build
Sponsor: jpawlowski@
Change-Id: I32244bc541f085ddd31ef1c85bd1f0b58b609f85