Revert "use utf8_length() instead of local function, isValidUtf8()"

This reverts commit 1a8b6c29aaa5a1591097daca0876808cc029cbda

Reason for revert: utf8_length handles a slightly different range than the checkjni code, so it's possible for a given string to pass the utf8_length check, but then fail the jni  check.

Change-Id: I81e15c95edd8f89782d8ad9025e68502b9340f22
Revert "makefile changes for gcm javadocs"

This reverts commit 5f9922d7c3bce158e4c7a58929d4075e7c91e32e

Change-Id: I310f27b231c6d64d6d407ae451b189a6ecf6b34f
KeyChain: return null instead of throw

The API documentation says it will return null if the key isn't found.
We get null back from the keystore daemon when it can't retrieve the
data, so just return null back to the API caller.

Change-Id: I42248bd50cbc5f76864bd762aae3faab1c50529d