[MediaProjection] Store WindowContext in field

When WindowContext is a local variable for starting layer mirroring,
the variable is eventually garbage collected. When this occurs
when MediaProjection is still running, then DisplayContent
fails to transform the captured content from
onConfigurationChanged.

The destroyed WindowContext triggers unregistering the
WindowContainerListener. Even though the WindowToken is
still being used by DisplayContent, DisplayContent is no
longer able to retrieve the WindowContainer associated
with the WindowToken. The WindowToken set when layer
mirroring begins is used to indicate the layer of
the hierarchy to mirror.

Bug: 197579637
Test: Manual
Change-Id: Ib8a971d1a85ebfe9db59d9267cc387f4f4d15991