Fix "preload-emulator" command

removed some commands that assumed the emulator was running, and weren't needed either way

stopped doKillEmu() blowing up
return immediately in some situations where adb blocks for a device
add adbSetTarget and adbInitialTarget
check validity of android target before running adb
run ndk-build from jni directory

this causes my jni builds to succeed where they would not previously, but ymmv
update readme