Show a Stopwatch widget in communal blueprint

This change introduces a keyguard section for widgets in communal mode,
and adds a Stopwatch widget to it, laid out at the bottom right corner.

Bug: 288275889
Test: adb shell cmd statusbar flag widget_on_keyguard on && \
      adb shell setprop ctl.restart zygote && \
      adb shell cmd statusbar blueprint communal
Test: atest KeyguardWidgetRepositoryImplTest
Test: atest CommunalInteractorTest
Test: atest DefaultCommunalBlueprintTest

Change-Id: Ib8a5a18bd78778c0915ef1796c594e170f2a0b64