WebView: relax URLUtil#isFileUrl()

This relaxes URLUtil#isFileUrl() to accept a prefix of "file:" instead
of requiring "file://". Chromium treats "file:path" as if the user had
typed "file:///path". An app may incorrectly believe it's safe to load
anything that URLUtil#isFileUrl() rejects, intending to refuse all URLs
which point to the file system.

Bug: 72848579
Test: cts-tradefed run cts -m CtsWebkitTestCases -t android.webkit.cts.URLUtilTest#testIsFileUrl
Change-Id: I7b3bba961523d4bdda6169a0f50fe7f1d1579e38