Only set USE_ANDROID_NDK_CPU_FEATURES if it's not already been explicitly set

R=djsollen@google.com, reed@google.com
BUG=skia:

Author: george@mozilla.com

Review URL: https://codereview.chromium.org/189263015

git-svn-id: http://skia.googlecode.com/svn/trunk@13750 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed