Drop deprecated libcore.io.Base64 in favor of java.util.Base64

Move the last last remaining user (DropBox) of l.i.Base64.encode()
to j.u.Base64.getEncoder().encodeToString() which produces exactly
the same result.

The decoding behavior of the two implementations differ:
 - in case of error, l.i.Base64 returns null vs. j.u.Base64 throws
 - j.u.Base64 is more strict about forbidding extra in-stream or
   trailing characters
 - the two implementations differ in how many bytes they decode if
   the encoded data ends prematurely.
To document the behavior change, the old libcore.io.Base64Test was
kept with updated expectations and under a new name.

Bug: 31292683
Test: libcore.java.util.LibcoreIoDerivedBase64Test

Change-Id: I62d4731d38619629d72549430c57e07f8ea4aa1e