Grant SharedStorageBackup storage access.

Shared storage backup had been relying on the WRITE_MEDIA_STORAGE permission to access
files on external storage, but that permission has been locked down,
and we need to use the real WRITE_EXTERNAL_STORAGE permission now.

Bug: 78938482
Test: build && boot && 'adb backup -shared' && 'adb restore backup.ab'
Test: make gts -j32 && gts-tradefed run gts -m GtsPermissionTestCases
Change-Id: Id244a1178936e8ffcccc7f95bcb120809a4b4607