Docs: clarify WebView support for cleartext traffic

No change to logic, docs change only.

WebView added support for android:usesCleartextTraffic for apps
targeting O and above (API level 26). This CL clarifies WebView's
support in the Android documentation.

This also fixes a preexisting presubmit error in
NetworkSecurityPolicy.java (unused import).

Bug: 67714197
Test: N/A
Change-Id: If6bfd36bc65926a1b032813598c85146ccfde969