Adding unique task affinity for FallbackHome activity

Settings activities were launched and resided on home task
which was originally started by FallbackHome. Adding a
unique task affinity for FallbackHome activity in order to
prevent the task being reused by other Settings activities.

Bug: 135696366
Test: starting Settings activity before FallbackHome finishes
Change-Id: I3fe41dd3b77e37236b11006dbf08727783b6a2ec

AndroidManifest.xml

@@ -2271,6 +2271,7 @@
                   android:excludeFromRecents="true"
                   android:label=""
                   android:screenOrientation="nosensor"
+                  android:taskAffinity="com.android.settings.FallbackHome"
                   android:theme="@style/FallbackHome">
             <intent-filter android:priority="-1000">
                 <action android:name="android.intent.action.MAIN" />