Improvements to Field.get/set.

Avoid unnecessary repeated computation in Field.get/set.
Refactor FromReflectedField and FromReflectedMethod into common helpers in
mirror::ArtField and mirror::ArtMethod, and make use of thereby avoiding
transitions through JNI.
Avoid JNI use from within FromReflectedField and FromReflectedMethod.
Tidy up Field.get/set wrt moving collector support.
Bug: 12189533

Change-Id: I643ab3474bade4abac3a3ae2b6e373b2bb0891c8
10 files changed