libnativebridge: Don't set deprecated CPU_ABI prop

CPU_ABI and CPU_ABI2 fields of android/os/Build are deprecated since
API 21. It should be safe to simplifiy the code and not touch them now.

We should also be able to stop overriding os.arch in java/lang/System,
but for this one I'd rather do more testing first.

Flag: EXEMPT NDK
Bug: 349529217
Test: tree-hugger
Change-Id: Ie25a648231097278d75c21a2d4b1cd4eb1984071