Fix bug where layers aren't shown in SurfaceFlinger dump.

The change in ag/15551285 clears the layersProto object when swap is
called. When the layers attempt to dump as a string, the variable is now
empty so nothing is shown.

Fixes: 199307267
Test: adb shell dumpsys SurfaceFlinger
Change-Id: I8c499352ffae086ef1264a6cf0c3df82e7a20977