[private_metrics] Increase resources for fetching the encryption key

This commit fixes an issue with fetching of the encryption key. The
fetch was previously returning NET_ERROR(INSUFFICIENT_RESOURCES, -12).
The maximum allowed size is being increased to make sure the key is
fetched successfully.

Bug: b:445106658
Change-Id: I8530f4454a285b1c986a3146ef4fe45c9888ffdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7092073
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Jay Zhou <zhouzj@google.com>
Commit-Queue: Chirag Arora <heychirag@google.com>
Cr-Commit-Position: refs/heads/main@{#1540682}
NOKEYCHECK=True
GitOrigin-RevId: bf170248a1bf07c8e2e63b9550131d43266141f6