Rename requestOptimizedExternalStorageAccess to more precise name

When apps use requestOptimizedExternalStorageAccess flag, they get
additional performance benefit by bypassing database operations. They
still have to scan the file to update MediaStore collection otherwise
MediaStore collection can go inconsistent with lower file system.
Current flag name could lead to apps making wrong assumptions hence
changing the name to more precise name.

Bug: 178209446
Test: atest packages/providers/MediaProvider
Change-Id: I00747da9ea53566cbcdd8aa8b2195e536b397b67