ANDROID: Move NDK_TRIPLE to build.config.constants.

... so that they can be loaded by Kleaf extensions
and read during the loading phase.

Moving forward, we should remove build configs in
the future and express constants in .bzl files. However,
for now, until kernel_build has been migrated to
use the defined cc_toolchain, we must keep this file.

Test: Treehugger
Bug: 228238975
Change-Id: Id9628663785970c460470382e1ae162e1112203d
Signed-off-by: Yifan Hong <elsk@google.com>