Rename fscrypt_is_native() to IsFbeEnabled()

Now that emulated FBE is no longer supported, there is no longer any
distinction between native FBE and emulated FBE.  There is just FBE.

Referring to FBE as "fscrypt" is also poor practice, as fscrypt (the
Linux kernel support for filesystem-level encryption) is just one part
of FBE, the Android feature.

Therefore, rename fscrypt_is_native() to IsFbeEnabled().

Bug: 232458753
Change-Id: Idf4cb25d37bc3e81836fcc5a1d96f79ccfa443b7