Set $HOSTNAME in adbd.

bash sets this itself, but mksh doesn't.

This also makes interactive and non-interactive adb shells more similar,
because when we set this in mkshrc that wouldn't affect non-interactive
shells. (I'm not sure whether this is an improvement or not.)

This is certainly _cheaper_ than doing this in mkshrc by calling out to
getprop though.

Bug: N/A
Test: `adb shell printenv` and `adb shell`, `printenv`
Change-Id: I3ff724f19a5098313df83836253f1f7e7872d6a4