[Android] Expose PDF flags as a setting

[timchilds]

Description

The latest chromium canary has two new flags available to enable an inline PDF viewer. This has been a user request for a while. It might also be useful for Leo to handle prompts on PDFs. We should expose these flags as a new setting.

User request examples

• Include PDFium Library on brave android to open pdf inside browser  #40808
• Can't open/download pdf(maybe other formats too) from google/brave search(probably other too)when site is http. #34125
• [Feature Request] Need option to open PDF in Brave Android app instead of auto-download #27855
• https://community.brave.com/t/how-do-i-open-pdf-files-into-brave-without-downloading-it-in-the-first-place/570121
• https://community.brave.com/t/pdf-reader-extention-built-in-browser/366488
• https://community.brave.com/t/read-pdf-files-without-keeping-them/120693

Requirements
https://bravesoftware.slack.com/archives/C03MMJ4CHEZ/p1733168133167609