lisa.analysis.idle: Make the CPU idle signal boolean

It really just is a boolean signal (on or off), and let's just say not
doing this is quite wasteful:

```
143.850158    0
143.850233    1
143.850378    0
143.854415    1
143.854635    0
dtype: int64
```

```
143.850158     True
143.850233     True
143.850378    False
143.854340    False
143.854415     True
dtype: bool
```