Remove legacy frames from WindowFrames and DisplayFrames

These frames are used for computing the legacy insets which won't be
used anymore. We should only maintain the new insets system now.

This CL also unhides Rect#inset APIs.

Bug: 149813814
Test: atest WindowFrameTests DisplayPolicyLayoutTests InsetsPolicyTest
            SplashscreenTests ManifestLayoutTests WindowMetricsTests
            WindowInsetsAnimationImeTests WindowInsetsControllerTests
            WindowUntrustedTouchTest DisplayContentTests
Change-Id: I06e40be6342b2ae35f7cc3e6f4ebdbe68edf0499