[PDF Ink Signatures] Use InkLoadedTextId in PdfInkModuleClient

Update PdfInkModuleClient and PDFiumEngine to use InkLoadedTextId for
loaded text annotations, separating them from newly created text
annotations (InkTextId).

Simplify LoadTextAnnotationsFromPdf() by removing the ID generator
callback, as the engine now generates InkLoadedTextId internally.

Bug: 514729749
Change-Id: Ie73ad4a2973ba346708994c90115192e388b6d34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7899781
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1643390}