fix(@angular/pwa): use relative paths in webmanifest

[cyrilletuzi]

Use relative paths instead of absolute paths in manifest.webmanifest, so PWA (including the service worker) can work with apps with base-href different from just /. It's especially important since Angular CLI v9 as the new --localize option uses base-href values different from just /.

Fixes #16922
Fixes #15092

[dgp1130]

Also merged to 9.0.x patch branch.

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