Don't force client composition when rounded corners are cached

Also tweak client composition summary characters so that all composition types correspond to a unique character.

Example with a few freeform windows open
Before: Layers: 19 [b:brrrr]r[R:rrrrrr]R[R:bbb]
After:  Layers: 19 [b:brrrr]r[r:rrrrrr]R[r:bbb]
There are 2 fewer client composited layers.

See go/shadow-caching for more examples.

Bug: 391428079
Test: Open a few freeform windows and take a perfetto trace. Ensure layers with rounded corners are being
Flag: EXEMPT bug fix
Change-Id: I88752bd6e03f531d3c7ed004cc085a255c4dc67c