Exempt the default browser from trampoline restrictions

After conversations with Chrome team, it was decided that we would
exempt the default browser from trampoline restrictions until T on the
grounds that they have to migrate the web ecosystem for a proper
long-term solution and couldn't avoid targeting the latest SDK.

Their transition plan is on: https://docs.google.com/document/d/1ptX0057LGTGCtTT_a3Gi2_MMyh8Pw-FM8aiRH4IWdVs/edit

Test: CTS coming
Test: atest com.android.server.notification.RoleObserverTest
Bug: 185460438
Change-Id: Ie86060c9f202776cb24bb5a58eb15bb7722d090e