For those still interested in using their P9, I've recently done manual updates to mine (since Huawei's OTA updates just never materialised).
I'm still running official Huawei software (EMUI5) but using various methods, I manually updated to Android 7 directly from Huawei's servers. I'm currently on C432B390 (C432 refers to the source of my handset - an EU unlocked P9 with no carrier - and B390 is the current latest update from August, since we're still waiting for the September BlueBorne security patch).
I've had no issues with Android 7 and DJI Go or Litchi, but YMMV if you've got a carrier-supplied handset (since software does vary).