Storage API polishing.

Based on API council feedback, switch to using real UUID objects
instead of Strings.  Since UUID is a general-purpose utility class
that will be passed around quite a bit, add it to Parcel and Bundle.

Define well-known namespaced UUID values for "default" and "primary
physical" storage devices, which will let us annotate a bunch of
things with @NonNull.

Define new extras for MANAGE_STORAGE intent that apps can use to
signal where and how much space they'd like the user to free up.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Bug: 37325923, 35812899, 35806020
Change-Id: I8421b126d680f69141a361c1e77223fe2bf4a325