Refactored TrustAgentUtils to allow agent check injection.

Notes:
- TrustAgentUtils defined a method that allowed callers to determine if a Trust Agent package is allowed to provide trust. If it is, it adds it to the list of available trust agents that can be displayed in the Settings->Security screen.
- The logic used to define what agents are permitted on the device can now be injected.

Test: Ran 'm RunSettingsRoboTest' and added a new test for the TrustAgentFeatureProviderImpl.
Bug: 34354635
Change-Id: I24c54c14bde26073ce6fa907379b86aae2841600

src/com/android/settings/security/SecurityFeatureProvider.java

@@ -19,6 +19,7 @@ package com.android.settings.security;
 import android.content.Context;
 import android.support.v7.preference.PreferenceScreen;
 
+import com.android.settings.trustagent.TrustAgentManager;
 import com.android.settingslib.drawer.DashboardCategory;
 
 
@@ -28,4 +29,7 @@ public interface SecurityFeatureProvider {
     /** Update preferences with data from associated tiles. */
     void updatePreferences(Context context, PreferenceScreen preferenceScreen,
             DashboardCategory dashboardCategory);
+
+    /** Returns the {@link TrustAgentManager} bound to this {@link SecurityFeatureProvider}. */
+    TrustAgentManager getTrustAgentManager();
 }