Suggest building at Android API level 16.

Android >= L requires that binaries be position independent. However,
this is only enabled with API level 16 or above. Since developers are
likely to have modern Android versions, suggest API level 16 in the
BUILDING file to save someone in the future having to figure this out.

Change-Id: I66db7228e3d6fef0aa8dcfcfff67a71cb630a2b9