Add two APIs in SettingsLibActivityEmbedding

Add two APIs

1. isActivityEmbedded() knows whether current activity is
embedded in the Settings app or not.
2. shouldHideBackButton knows whether current activity is
suggested to show back button or not.

Bug: 219000314
Test: Use the api in Settings app, and behavior is correct.
Change-Id: If1d29a8d9bdb896b179275e13de649872a5b94fe