[DocsUI M3] Update alpha for hover/ripple

Update the alpha value for hover and ripple layer to match
the material spec:
* hover - 8%
* ripple - 10%

Bug: 388127521
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I0078d4e28d2e55fd97ab1aa26e7532d7664ef928