Trapping backspace at EditText level to prevent popping Docs.

When the EditText is empty, pressing backspace on soft/hardware keyboard
will call on the EditText's OnKeyListener. Since we don't handle
backspace, it will then bubble up to Activity-level, which will then pop
the document stack due to the feature of using backspace as navigation.
This will trap the call correctly if the EditText is empty.

Bug: 30066261
Change-Id: I231507b9469bfa52478872491f1d2713a1ac58ba