Make getExternalStorageMountMode as SystemApi

Secondary volumes are FUSE mounted, whereas Android/data and Android/obb
on primary volumes are not FUSE mounted. Access to these private app
directories on primary volumes is restricted using the mount modes, but
access for these on secondary volumes need to be regulated by
MediaProvider.

Make getExternalStorageMountMode as SystemApi so that MediaProvider
can leverage the same mount logic for Secondary volumes.
Expose relevant mount modes as SystemApi as well.

This change saves us the maintenance overhead for Secondary volumes for
Android S+. Otherwise we would have to check if an app is a signature
app through APIs which would basically be the duplication for the
logic in StorageManagerService.java.

Expose ExternalStorageProvider and downloads Authority for MediaProvider
to rely only on APIs. This is also required for CDD modifications that
these 2 providers are given exceptional access to private app dirs.

Bug: 175796984
Bug: 173505864
Bug: 173505864
Test: atest DownloadProviderTest
Test: atest CtsScopedStorageHostTest
Test: atest android.scopedstorage.cts.host.PublicVolumeHostTest#testCheckInstallerAppAccessToObbDirs
Test: atest android.scopedstorage.cts.host.PublicVolumeHostTest#testCantAccessOtherAppsExternalDirs
Change-Id: I51bc7bd3f355fadd9de56ac267469c2352eb0ffa
Merged-In: I51bc7bd3f355fadd9de56ac267469c2352eb0ffa