doc-centric compose

use 'intoExisting' so repeated taps on Reply for a particular message
goes right to any in-progress compose.
use 'autoRemoveFromRecents' because nobody wants stale Compose tasks
accessible from Recents.

Change-Id: Icb39b504f51d20c8c2a11105c1af7a8b371234c4

AndroidManifest.xml

@@ -90,6 +90,8 @@
         <activity
             android:name="com.android.email.activity.ComposeActivityEmail"
             android:label="@string/app_name"
+            android:documentLaunchMode="intoExisting"
+            android:autoRemoveFromRecents="true"
             android:theme="@style/ComposeTheme">
             <intent-filter>
                 <action