Add an __ANDROID_RAMDISK__ preprocessor define.

There is no easy way to inform C++ targets whether they're building "not
in normal Android". In the past we used __ANDROID_RECOVERY__, but this
isn't defined for ramdisk targets, or for vendor_boot. Furthermore
there's no "target.ramdisk.cflags" option to add it.

Add a new __ANDROID_RAMDISK__ define that will be set in any ramdisk or
recovery target (since recovery by definition is in a ramdisk).

Bug: 205987817
Test: mm bootimage, vendorbootimage, recovery
Change-Id: If3cd34b07fe0e20abc5108cb338a56698d33e42e