Deprecate USER in favor of BUILD_USERNAME

# Build System Changes for Android.mk Writers

## `USER` deprecation  {#USER}

`USER` will soon be `nobody` in many cases due to the addition of a sandbox

around the Android build. Most of the time you shouldn't need to know the

identity of the user running the build, but if you do, it's available in the

make variable `BUILD_USERNAME` for now.

Similarly, the `hostname` tool will also be returning a more consistent value

of `android-build`. The real value is available as `BUILD_HOSTNAME`.

## `BUILD_NUMBER` removal from Android.mk  {#BUILD_NUMBER}

`BUILD_NUMBER` should not be used directly in Android.mk files, as it would

  GLOBAL_CFLAGS_NO_OVERRIDE GLOBAL_CPPFLAGS_NO_OVERRIDE \

  ,GCC support has been removed. Use Clang instead)

$(KATI_obsolete_var DIST_DIR dist_goal,Use dist-for-goals instead. See $(CHANGES_URL)#dist)

$(KATI_deprecated_var USER,Use BUILD_USERNAME instead. See $(CHANGES_URL)#USER)

# This is marked as obsolete in envsetup.mk after reading the BoardConfig.mk

$(KATI_deprecate_export It is a global setting. See $(CHANGES_URL)#export_keyword)