[adb] Issue the "auth" emulator command before any other one

Emulator console now requires authentication; this means
'adb emu ...' commands silently fail because of it.
This CL adds an 'auth <token>' command to each user command,
making sure it won't be silently ignored.

Bug: https://code.google.com/p/android/issues/detail?id=211233
Change-Id: Id9ca4999fd2e6393cc88278eaf444243e13c0ec0