Add a TesteeHandlerThread to make async test call easier.

Define a customized HandlerThread which notifies main testing
thread when it's done processing all messages. So that main
testing thread doens't need to wait a fixed amount of time which
slows down testing.

Bug: 138886216
Test: unittest
Change-Id: Ic932345ada856b8d49bb7cf0929c44ff90d394c8