Fix filenameFormControl issue.

makeDownloadArchiveFilename contained no checks to make sure the returned string was valid according to FileUtils.DOWNLOAD_FILENAME_REGEX.
Introduced these checks, and also initialised filenameFormControl in AppComponent to avoid future issues with it not being defined in places.

Bug: b/309431531
Test: npm run test:unit:ci
Change-Id: Ic996b18d92b65bf937a87a4630e08a0d6e7fc024
5 files changed