Define __ANDROID_API__ for all Android builds.

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

This macro (along with __ANDROID__) should always be defined for Android
targets. We set it to the major (only) version of the Android API being
compiled for. The Android version is able to be set as an integer suffix
for any valid Android target.

Reviewers: danalbert, eugenis

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

Differential Revision: https://reviews.llvm.org/D26385

llvm-svn: 286295