Disable ndk default features to remove raw-window-handle 0.6 (#142)

The `ndk` crate enables `raw-window-handle 0.6` by default (because of
rust-mobile/ndk#434 (comment))
which might not be used by consumers of the `android-activity` crate
at all, or might (still) be a mismatching version. Even if the `rwh_0x`
features are additive, figuring that out leads to cryptic errors and it
is best to turn off these defaults completely and leave it to the user
to turn it back on in their own `[dependencies]` section if desired.

Author: ThierryBerger

android-activity/Cargo.toml

@@ -34,7 +34,7 @@ jni-sys = "0.3"
 cesu8 = "1"
 jni = "0.21"
 ndk-sys = "0.5.0"
-ndk = "0.8.0"
+ndk = { version = "0.8.0", default-features = false }
 ndk-context = "0.1"
 android-properties = "0.2"
 num_enum = "0.7"
@@ -54,3 +54,6 @@ targets = [
 ]
 
 rustdoc-args = ["--cfg", "docsrs"]
+
+[dev-dependencies]
+ndk = "0.8.0"