build: Fix Android host system checks for cross compilation

When building for Android, host_system is "android-$abi", for example 'android-x86_64'. host_system == 'android' would not detect this, because 'android' != 'android-x86_64'.

@oleavr, I assume you (cross,)compiled differently than I did, but my change would still cover your use case. 'android'.startswith('android') may look inefficient and ugly, but it still returns true, so your build shouldn't break.

Intentionally left out a check in meson.build#L345 from this merge request, because it is being removed in !1874 (closed) .