x/mobile: gomobile doesn't find Android NDK installed from typical dist pkgs

[quite]

On Ubuntu, the google-android-ndk-installer leaves the NDK in /usr/lib/android-ndk, and it seems to be up to the user to set ANDROID_NDK_HOME.

On Arch Linux, the android-ndk package installs the NDK into /opt/android-ndk and sets ANDROID_NDK_HOME to there using an /etc/profile.d/file

There are also other variables, such as ANDROID_NDK, but they seem deprecated. This was somewhat informative: openssl/openssl#8103

[gopherbot]

Change https://golang.org/cl/171938 mentions this issue: cmd/gomobile: look also in $ANDROID_NDK_HOME for the NDK...

[eliasnaur]

One complication is that gomobile build uses tools from the Android SDK. What to do when only ANDROID_NDK_HOME is set?

[quite]

It looks like all code that references ANDROID_HOME (which is where such tools must be picked up) returns error if it can't be found.

And the code that I touch for this issue is only in the ndkRoot() function, so just doing our best there to find the NDK should be enough.