Zero out framenumber when clearing out layer

Fixes a bug where the contents of the client composition buffer
was not reused because HWC requested clearing the layer. When
checking if the composition request changed, the framenumber was
always incrementing.

Bug: b/146238164
Test: go/wm-smoke
Test: adb shell su root dumpsys SurfaceFlinger --timestats -dump -maxlayers 0
Change-Id: I39029835003b40c7e50acf765b52ee9103ce8303