add test for constant expression evaluation output

Simply run:

    # hidl_gen converts .hal to .h correctly
    make hidl_gen_test
    # converted .h can compile
    make android.hardware.tests.expression@1.0

to run the tests.

b/30951879: int8_t is same as char, and when emitting output the
	    character corresponding to that number is emitted instead of
	    the number itself.

Change-Id: Icacba6b6262cc0026a94f64527b80e2c0e35e72e