Define __ANDROID_API__ when specified as part of an Android target.

Summary:
This macro should be defined only when the user directly specifies an
API level as part of an Android target. For any regular Android target,
we leave this macro undefined.

Bug: https://llvm.org/bugs/show_bug.cgi?id=30940

Reviewers: eugenis, pirama

Subscribers: tberghammer, cfe-commits, pirama, eugenis, danalbert

Differential Revision: https://reviews.llvm.org/D26491

llvm-svn: 286543