FBE devices now fully support adoptable storage.

We've finished all the underlying work to support adoptable storage
on FBE devices, so remove the code that was disabling it by default.

Introduce feature flag to make it easier to detect devices that
support adoptable storage.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.AdoptableHostTest
Bug: 29923055, 25861755, 33252673, 37289651
Change-Id: I3630d690c9e66c7e41e316a4263ea2eb1e752ad3