Fixes for tetheroffload crashes

Now that we can talk to the HALs (with some out of tree CLs and
"setenforce 0"), several crashes were encountered.

Fixes here include:
    - avoid hidl_handle move semantics
    - check HIDL method status return value (isOk())
    - convert Java short port numbers to ints
    - don't pass nulls to HIDL where Strings are required
      (limitations in parceling)

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
    - "setenforce 0" and start tethering
Bug: 29337859
Bug: 32163131

Change-Id: I91314440c3a04e5f2502579b5f06dac9f25cf0cd