WebView: require APKs to target OMR1

This disallows WebView/Monochrome APKs targeting O or below to be chosen
as WebView providers.

Bug: 65080651
Test: Open WebView shell with webview from system image, no crash
Test: Install Monochrome compiled for O, it's not in the webview provider list
Change-Id: I79162cedaa801ce227ad4c2eb3cbea9c08704da8