Add the CertificateTransparencyDownloader to download the data and
metadata needed by Certificate Transparency.
The download is triggered when either the content URL, the metadata URL,
or the log list flags change.
The version numbers and URLs are stored in an on-disk DataStore to make
sure the same files are not downloaded on repeated flag updates.
Flag: com.android.ct.flags.certificate_transparency_service
Bug: 319829948
Test: atest NetworkSecurityUnitTests
Change-Id: Ic30e2973fdce99ea861d13f7777128161dd4003e
799860a21d