JNI: Fast-path for decoding returned jobject.

Results for the timeGetBytesAscii#EMPTY benchmark from the
libcore's StringToBytesBenchmark suite on blueline-userdebug
with the cpu frequencies fixed at 1420800 (cpus 0-3; little)
and 1459200 (cpus 4-7; big):
  32-bit little: ~415 -> ~390
  64-bit little: ~415 -> ~390
  32-bit big:    ~180 -> ~170
  64-bit big:    ~180 -> ~170

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing --debug --ndebug
Test: run-gtests.sh
Test: testrunner.py --target --optimizing --debug --ndebug
Bug: 172332525
Change-Id: I0e19d583e5141e99a8b8c6fd9ae125fe7c9e02e7
12 files changed