Use unique_ptr for proto parser to handle destruction.

There's a memory leak where Layers created for the proto tree are not
removed when no longer used. This ensures that there's a unique_ptr for
each parser layer created so it will be automatically destroyed when
out of scope.

Change-Id: I56731d28a39e7d9d157c59065102d97f316f3b7d
Fixes: 74071380
Test: adb shell dumpsys SurfaceFlinger
Test: LayerProtoStress.mem_info no longer shows increased memory