Bugfix: Java char is 16 bits, can not be treated as boolean.

Using SetFieldBooleanVolatile() and SetFieldBoolean() happens to work
for char values that only use the lower 8 bits, but is a mistake that
was introduced by the "Add AccessibleObject and Field to mirror" commit:

https://android.googlesource.com/platform/art/+/daaf326

Change-Id: Iec02ba3084c992ea239ecef688d7d29c7e21ae16