Make signapk use Conscrypt.

This makes the signapk tool use Conscrypt (where possible) instead of
the platform-default JCA providers and the Bouncy Castle JCA provider.
This speeds up (by 10-30%) APK and OTA update signing because
Conscrypt's crypto primitives are backed by BoringSSL.

Previously, the signapk tool consisted only of the signapk.jar.
Because Conscrypt is backed by native code, signapk now consists of
signapk.jar and crypto_openjdk_jni shared library. This requires that
users of the tool be updated to provide a suitable -Djava.library.path
argument to the Java runtime. This change updates all known users of
the tool inside the Android source tree to do so.

Bug: 26097626
Change-Id: I8411b37d7f771ed99269751a3007dff103083552