android: capture current keyboard height

expose a new virtual_keyboard_height function under notedeck::platform::android

which gets the current height of the virtual keyboard. We can use this
to tranlate the view out of the way

Signed-off-by: William Casarin <jb55@jb55.com>

Cargo.toml

@@ -61,6 +61,7 @@ sha2 = "0.10.8"
 bincode = "1.3.3"
 mime_guess = "2.0.5"
 pretty_assertions = "1.4.1"
+jni = "0.21.1"
 
 [profile.small]
 inherits = 'release'