adb: win32: fix adb shell

28416d6d broke adb shell on Windows
because it was calling unix_read() with a buffer size of 1 byte which
wasn't supported by unix_read()/_console_read().

This change makes _console_read() support such a small buffer by
maintaining an internal input buffer.

This lets us simplify the existing code that was already maintaining a
cached INPUT_RECORD.

Also eliminate some duplicate code in the big switch statement.

Change-Id: I8f0aed4fb9f6f2f5b9a6b68ce60d2e368fec81c7
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>