Implicitly expose activities

Activities that have a browsable web intent filter [action.VIEW,
category.BROWSABLE and http(s) scheme] are implicitly exposed to
instant apps. Authors can explicitly hide these activities by
using the attribute android:visibleToInstantApp="false".

Bug: 25119046
Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.EphemeralTest
Change-Id: I88b277a719e7ecd5631f8cbd281757eb3e3d2ce6