[FilesBanner] Implement showLimit functionality

The showLimit defines the number of times a banner is shown in units of
Files app sessions. A session is defined as a new foreground window
being opened up (i.e. a new instance of BannerController being setup).

Bug: 1228128
Test: browser_tests --gtest_filter=FileManagerJsTest.Banner*
Change-Id: I1c8a7805a9890a533d544f5a1fbb93bf584e2326
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3058330
Commit-Queue: Ben Reich <benreich@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908272}