Use proper names for Android NDK and SDK locations

This bug may already be out of date as it looked like ANDROID_NDK_HOME was being phased out, but I wanted to point this out because it looks like it still exists in the code base and I believe the variable is wrong.

If anything, it should be ANDROID_NDK_ROOT, not ANDROID_NDK_HOME. Last time I pinned the Android folks down on these, they didn’t want to say any of these were official, but this is the explanation:

This was from David Turner from the Android team:
[quote]

ANDROID_NDK_ROOT -> installation dir of NDK
ANDROID_SDK_ROOT -> installation dir of SDK
ANDROID_SDK_HOME -> location of SDK-related user files, defaults to
~/.android/ on Unix.

These variables are essentially used by the platform and SDK tools
themselves, so are your best bet.

There is no ANDROID_NDK_HOME.

They are also used by the Chromium build for Android.
[/quote]

If Swift is going to continue using a variable, ANDROID_NDK_HOME should be changed to ANDROID_NDK_ROOT.