Fix KeyCharacterMap overlays by reloading base overlay before applying another overlay.

Add all fields to parcel (mLoadFileName, mLayoutOverlayApplied, mKeysByScanCode, mKeysByUsageCode).
Add all fields to equality operator.
Add tests to libinput_tests.

Bug: 210090260
Test: Manually verified that layout changes correctly.
Test: atest libinput_tests
Change-Id: I1777b87e5c2ce8a0dbd42ad48748ea9e669c9139
(cherry picked from commit 90bc9490)