Convert all Log.* messages in KeyguardUpdateMonitor to use LogBuffer

Create a new helper class for logging.

Current messages being logged to Logcat won't be logged anymore, the
LogBuffer logs can be accessed by a dumpsys.

For debugging, LogBuffer can configured to echo to Logcat as well
using the following command:

```
adb shell settings put global systemui/buffer/KeyguardUpdateMonitorLog
<LogLevel>
```

Test: atest SystemUI passes
Fixes: 239819049
Change-Id: I8f234f930d1aab92ae3535d8f850d26c2eab6729