Autofill SaveUi URL validation and FLAG_IMMUTABLE

This change enhances the security of the Autofill Save UI by strictly enforcing http and https schemes for custom description links, preventing custom scheme hijacking. Additionally, it switches the PendingIntent created in SaveUi to be immutable (FLAG_IMMUTABLE), preventing potential intent manipulation by recipient applications.

Bug: 465446148
Test: Verified with PoC test app
Test: Verified with customized autofill test app
Test: atest CtsAutoFillServiceTestCases
Flag: EXEMPT CVE_FIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:e23397d2369608c73452d2c10149ff24bdc79bcc
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:c6d564a4c54677ff785e23981f5f4c7cc4a5fb22
Merged-In: I32f07cac574a1073af13c04a789ff06f6a0ab90a
Change-Id: I32f07cac574a1073af13c04a789ff06f6a0ab90a