Add elevation to workFAB to create a shadow

Unable to use md.sys.elevation.level3 but codesearched and
looked it up that this is equivalent to 6dp.

bug: 375597648
Test:
before: https://drive.google.com/file/d/1fp2l5p4wW2Gf3duC2qQ_brQ-xY0bTnOF/view?usp=sharing
after: https://drive.google.com/file/d/19xSAnWJURuwrjhMiywfjNT2vD0G2GoFP/view?usp=sharing
Flag: NONE adding elevation
Change-Id: I484609b4dc817b6d76701f60da51c3ec89bc8755

res/layout/work_mode_fab.xml

@@ -17,6 +17,7 @@
     android:id="@+id/work_mode_toggle"
     android:layout_height="@dimen/work_fab_height"
     android:layout_width="wrap_content"
+    android:elevation="@dimen/work_fab_elevation"
     android:minHeight="@dimen/work_fab_height"
     android:gravity="center_vertical"
     android:background="@drawable/work_mode_fab_background"

res/values/dimens.xml

@@ -155,6 +155,7 @@
     <!-- Floating action button inside work tab to toggle work profile -->
     <dimen name="work_fab_height">56dp</dimen>
     <dimen name="work_fab_radius">16dp</dimen>
+    <dimen name="work_fab_elevation">6dp</dimen>
     <dimen name="work_fab_icon_size">24dp</dimen>
     <dimen name="work_fab_icon_vertical_margin">16dp</dimen>
     <dimen name="work_fab_icon_start_margin_expanded">4dp</dimen>