Plumber: flushing handler threads doesn't seem to work on Android 8

[pyricau]

Example trace:

	
┬───
│ GC Root: Java local variable
│
├─ android.os.HandlerThread thread
│    Leaking: UNKNOWN
│    Retaining 3.8 MB in 42126 objects
│    Thread name: 'Sq-LCR'
│    ↓ HandlerThread.<Java Local>
│                    ~~~~~~~~~~~~
├─ android.os.Message instance
│    Leaking: UNKNOWN
│    Retaining 3.8 MB in 42115 objects
│    Library leak match: instance field android.os.Message#obj
│    Message.what = 67
│    Message.when = 0 (1681316 ms before heap dump)
│    Message.obj = instance @362438672 of com.squareup.container.inversion.DialogCoverViewKt$coverView$1
│    Message.callback = null
│    ↓ Message.obj
│              ~~~
├─ com.squareup.container.inversion.DialogCoverViewKt$coverView$1 instance
│    Leaking: UNKNOWN
│    Retaining 3.8 MB in 42112 objects
│    Anonymous class implementing android.content.DialogInterface$OnDismissListener
│    ↓ DialogCoverViewKt$coverView$1.$view
│                                    ~~~~~
├─ com.squareup.container.inversion.PosSheetContainer instance
│    Leaking: YES (View.mContext references a destroyed activity)