adb: improve names for transport threads.

The old names seems confusing. output_thread was reading remote data and writing to
local sockets. input_thread was reading local sockets data and writing to remote.
This change tries to make it clear by renaming output_thread to read_transport thread,
and renaming input_thread to write_transport thread.

Change-Id: I2e7b4cde7a94d436f3745e9e3ab10780e7caa8ac