[DocsUI M3] Add a string for "clear selection"

When files are selected, we will show a close button on the app
bar to clear selection, previously the content description for
this button is "Cancel", this CL creates a new string with
"Clear selection" to make it more explicit.

Bug: 408898634
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I17e7cd4b06729a1207178e6af0e30b2c503ef9fb