Add testkey_rsa2048.pem to ${SOONG_OUT}/dist/share/avb

This key can be used to sign a boot.img.

Bug: 176930600
Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: I9a1413768e170e1f189503a00cf3b1dbb8f31e3e