Use Activity resources in DecorContext

We use the application context in DecorContext to avoid holding a reference
to the Activity. However, the resources don't match the Activity's in a multi-window
world, so we use the resources from the Activity, which  get updated as the Activity
goes between multi-window states. This avoids holding a reference to the Activity.

Bug:28096052
Change-Id: I2804f2420afad6dcecc56e5dece008c390540d72