Keystore 2.0 SPI: Fix bug in chunked streamer.

The chunked streamer sent the chunk buffer prematurely leading to
oversized and garbage data sent to keystore.

Test: atest android.keystore.cts.SignatureTest#testSmallMsgKat
Change-Id: I84e40766b735f05b3fb7e0e692d26a25a0496649