Add missing default dirs to MediaProvider

MediaProvider creates the default directories when a new volume it
attached, and also ensures that no app can create a top level directory
if it's not a default directory. This means that any default directory
that MediaProvider doesn't know about cannot be created (by apps that
abide by scoped storage, at least). This change adds 2 missing
directories:
    * Audiobooks
    * Documents

Test: build
Bug: 150695960
Merged-In: I5b07176eb3821933f3f1d13345c5f642bb6dc8b7
Change-Id: I82224b016881fba74f34481ae0bf1f6c36f7059f
1 file changed