Replace __ANDROID_NDK__ with __ANDROID__

Summary:
This replaces all the uses of the __ANDROID_NDK__ define with __ANDROID__. This
is a preparatory step to remove our custom android toolchain file and rely on
the standard android NDK one instead, which does not provide this define.
Instead I rely, on __ANDROID__, which is set by the compiler.

I haven't yet removed the cmake variable with the same name, as we will need to
do something completely different there -- NDK toolchain defines
CMAKE_SYSTEM_NAME to Android, while our current one pretends it's linux.

Reviewers: tberghammer, zturner

Subscribers: danalbert, srhines, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D27305

llvm-svn: 288494