Use context-specific configuration signal when updating drag layout

- The configuration coming through this window context is not in sync
  with the app configuration change (what SysUI uses), so we should
  listen for this context callback instead to make sure we read the
  resource at the right time

Bug: 270446029
Test: atest WMShellUnitTests
Test: Repeatedly toggle dark mode and verify drag outlines update to
      dark/light colors
Change-Id: I2aa8abfd6d3607d0b54a37414db87522688d0dd5