Implement clearkey hidl `lshal debug` hook

The output of the hook can viewed using the command:
> adb shell lshal debug android.hardware.drm@1.3::IDrmFactory/clearkey

The hook can be used to emit debug info from the ClearKey hidl process.
Currently only emit the total number of open sessions.

Bug: 153663446
Test: adb shell lshal debug \
  android.hardware.drm@1.3::IDrmFactory/clearkey
Change-Id: Ie5918acbfbbb5cfc2a9bc5e138ab0320f3c6a558