[android] Include api-level.h for Android and C++ interop.

[drodriguez]

When the header was used in Android, the usage of __ANDROID_API__ was
not set if the compiler wasn't setting it externally. There was a check
for __ANDROID_API__, which defaulted to zero, and so it didn't pass. The
external function definition was not being done, but in C mode, it
didn't matter because implicit functions are allowed. That's not true in
C++ mode, which fails to compile any code that tries to include this
header.

The solution is including android/api-level.h which will define
__ANDROID_API__ to a very high value if it is not defined already (which
is the default behaviour in the NDK).

[drodriguez]

@swift-ci please smoke test

[pschuh]

This looks fine to me, but won't this just set it to __ANDROID_API_FUTURE__(10000) as per: https://android.googlesource.com/platform/bionic/+/master/libc/include/android/api-level.h ?

[drodriguez]

Yes. It does. That's the behaviour when compiling the NDK. If you don’t provide an API level, the build system understand that you are targeting the latest API level available. When this file is compiled as part of Swift, the API level is actually provided as 21, but since this is a test, the definition is not set.

I might look into changing the test invocations to provide the API level used during building the stdlib (since it might reflect better how Android is compiled), but this should be needed in case someone decides to compile for Android without providing their own value for __ANDROID_API__.