floss: Add partial BleScanner topshim

Add a partially implemented BleScanner topshim implementation. This
implements the callbacks but not the method calls into BleScanner since
lack of callbacks is causing a crash in Floss.

Bug: 216384386
Tag: #floss
Test: Discovery works without segfault on ChromeOS
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines

Change-Id: If843996a41d44e21ab436c1ceaf825971f6ce1a2