macros: Clean up the jni signature-checker code

Minor cleanups based on comments from previous commit.

Bug: 35325126
Test: make -j32 JniSafeRegisterNativeMethods_test \
  && out/host/linux-x86/nativetest64/JniSafeRegisterNativeMethods_test/JniSafeRegisterNativeMethods_test

Change-Id: I9aada9748677d02351988707249c9c5d0ead3597