NDK library: use prebuilts to check next level ABI

NDK has two ABI checks:

1. Check that prebuilt ABI exactly matches compiled binaries.

2. Check that ABI of level X compatible with level X+1.

The second check used compiled binaries for level X and prebuilts for
level X+1. But this approach may confuse people, because ABI change will
be shown "reversed". For example, adding new field to ABI monitored
binary of level X would be shown as "removed" when compared to level X+1
prebuilt.

This change uses prebuilts for both level X and level X+1. Given, that
prebuilts are checked to match compiled binaries, this should be enough.

Change-Id: If942e4319744bc5a2674cdd65f6a6f664fdfaa7e
Signed-off-by: Aleksei Vetrov <vvvvvv@google.com>