adb: refactor _is_valid_ack_reply_fd

Visual Studio's 'jump to reference' feature couldn't parse
adb_commandline() because I used an #ifdef in the middle of an if
statement, so this refactors the code into a separate helper function. I
just copied the code and inverted the comparisons.

No need for sysdeps since this is pretty minor.

Change-Id: Ifd5c62b0b505080ada6db5cc19739c6f07b94de9
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>