JDWP: update handling of 8-bit and 16-bit fields

We must use special getter/setter for types smaller than int:
boolean, byte, char and short. However, JDWP still requires to treat:
- int and float as 32-bit values
- long and double as 64-bit values

Bug: 21746739

(cherry picked from commit 05c26b30)

Change-Id: I2d667291974e191f2c9b10311ce02435b902dd0c