Make the demo compile with NDK

[floppyhammer]

Solving #472. Upgrade usvg to 0.20.0, which removes harfbuzz_rs dependency.

[floppyhammer]

The demo shows some artifacts on Android, which doesn't occur in the last release. I'll open another PR to handle that.

[jdm]

@bors-servo r+ Thanks!

[bors-servo]

:pushpin: Commit 9c1f48a has been approved by jdm

[bors-servo]

:hourglass: Testing commit 9c1f48aa411b33fb34b33ddd183d612b96a8c8e8 with merge db0dfee23812bcd6b1efea1f9e166698839d36d4...

[bors-servo]

:broken_heart: Test failed - checks-github

[jdm]

error[E0599]: no function or associated item named `from_file` found for struct `Tree` in the current scope
  --> utils/svg-to-skia/src/main.rs:16:22
   |
16 |     let tree = Tree::from_file(&input_path, &Options::default()).unwrap();
   |                      ^^^^^^^^^ function or associated item not found in `Tree`

[floppyhammer]

@jdm Should probably merge https://github.com/servo/pathfinder/pull/493 first.