Expose ActivityOptions#isShareIdentityEnabled API

Android U introduced new APIs to allow a launching app to share its
identity with a launched activity. This commit exposes a new API
in ActivityOptions to return whether a launching app has opted-in to
sharing its identity from an instance of ActivityOptions.

Bug: 262712655
Test: atest ShareIdentityTest
Change-Id: I6f77ed7151baa3ca572ef58a1867a91c98403a97