AndroidRuntime: when CheckJNI is enabled log a message

This matches behaviour described in:
 https://developer.android.com/training/articles/perf-jni#extended-checking

All output from CheckJNI state was disabled as part of quietening
logspam, but now we log a message only if CheckJNI is
enabled. CheckJNI is off by default and only used as a developer
correctness tool.

Bug: N/A
Test: adb root; adb shell stop;                        \
      adb shell setprop dalvik.vm.checkjni 1;          \
      adb shell start; adb logcat -d | grep CheckJNI
Change-Id: I93bc2d097a8769b7768c633b705b29fc1aac634a