[WebSignin][A11y] Make subtitle texts focusable in bottom sheet

This CL makes the subtitle texts of web sign-in bottom sheet focusable
in talkback mode.

Video demo: crbug.com/1159940#c1

Bug: 1159940
Change-Id: I571faecd0f26500a2c9a39336767bf8071926118
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595739Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838628}

chrome/android/java/res/layout/account_picker_bottom_sheet_header.xml

@@ -30,6 +30,7 @@
       android:layout_marginStart="24dp"
       android:layout_marginEnd="24dp"
       android:layout_marginBottom="16dp"
+      android:focusable="true"
       android:gravity="center_horizontal"
       android:textAppearance="@style/TextAppearance.TextMedium.Secondary"
       android:text="@string/signin_account_picker_bottom_sheet_subtitle"

chrome/browser/incognito/interstitial/android/java/res/layout/incognito_interstitial_bottom_sheet_view.xml

@@ -33,6 +33,7 @@
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:layout_marginBottom="16dp"
+        android:focusable="true"
         android:gravity="center_vertical"
         android:textAppearance="@style/TextAppearance.TextMedium.Secondary"
         android:text="@string/incognito_interstitial_message"