Return null on getPrivateKey failure not exception

According to documentation:

Returns the {@code PrivateKey} for the requested alias, or null if
there is no result.

@throws KeyChainException if the alias was valid but there was some
problem accessing it.

@throws IllegalStateException if called from the main thread.

In this case the alias doesn't exist or isn't visible to the caller so
they should get null back instead of KeyChainException.

Change-Id: Ied5603ac6aefbcef79050f24c2aa7ee8f386be0b