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
8eb783b21e