Stop writing NUL bytes in adbkey.pub.

In N we moved some code from C to C++ without realizing that EVP_EncodedLength
includes space for a terminating NUL and EVP_EncodeBlock writes one. Because
our key reading code copes with the NUL, we never noticed.

Distinguish between the required space returned by EVP_EncodedLength and the
actual number of bytes (not including NUL) used return by EVP_EncodeBlock.

Bug: http://b/36187819
Test: hexdump of ~/.android/adbkey.pub

(cherry picked from commit 0b771b33)

Change-Id: I6e16b8d48d097b4054417c1d1a225bf7ece985b9