ash: Visual and accessibility fixes for in-session auth dialog

1. Adjust title font size and margins according to style guide
2. Left align title, right align cancel button.
3. Apply round corners.
4. Enable title to be read out in accessibility use cases.
5. Enable fingerprint label to be read in accessibility use cases.

Bug: b:156258540, b:144861739
Change-Id: I2d5970da96e31f9df162ccbe74db8d95fdd49ec6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393249


Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yicheng Li <yichengli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805541}