Remove dead isolated storage code from StorageManagerService

Isolated storage was added in Q (and enabled by default) and has been
replaced by FUSE in R

Additionally, we can remove the ExternalStorageMountPolicy concept
which is only required if isolated storage is false

Cleanup in preparation to introduce DeviceConfig changes for
transcoding

Bug: 169327180
Test: Builds
Change-Id: Ie2686d8efa6654ea6c4b4ca8c121c0c1841a6356