UWP: non random port for devtools, and support for custom UWP prefs

[paulrouget]

packages-prefs.json is now taken into account for libsimpleservo. Making it possible to have custom pref for the UWP builds, removing some weirdness in the way we handle preferences in the hololens code.

This also adds a new set of preferences to control the devtools server startup state, and make the port choice constant across sessions.

Fix #27267
Fix #22970

[jdm]

This is a nice cleanup!

[paulrouget]

What about the args variable that is passed in?

Good catch. Fixed.

[paulrouget]

Hold on. I found a regression.

[paulrouget]

The way the homepage pref was overridden wouldn't work because we were relying on the fact that the UWP pref always had a shell.homepage key, which is not the case anymore as we use the servo pref.

Fixed with: 9174b20#diff-831a752eac48b390b35aae339dbc1097R375

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit b56dccb has been approved by jdm

[bors-servo]

⌛ Testing commit b56dccb with merge 8cf45f4...

[bors-servo]

💔 Test failed - status-taskcluster

[jdm]

error: unused import: `set_pref`
  --> ports\libsimpleservo\api\src\lib.rs:34:33
   |
34 | use servo::servo_config::{pref, set_pref};
   |                                 ^^^^^^^^
   |
   = note: `-D unused-imports` implied by `-D warnings`

[paulrouget]

@bors-servo r=jdm

[bors-servo]

📌 Commit 1b0de56 has been approved by jdm

[bors-servo]

⌛ Testing commit 1b0de56 with merge 774673d...

[bors-servo]

☀️ Test successful - status-taskcluster
Approved by: jdm
Pushing 774673d to master...