Support given content insets for non-IME windows

This will let the task bar and all other windows provide an insets frame
smaller than the window's frame. This is a regression caused by the
flexible insets change.

If the window provides both internal insets and the content insets, both
of them will be deducted. A comment is added to the
provideInternalInsets to avoid the mis-use. There's a long term plan to
change the functionality of the provideInternalInsets. Will have it in
the follow-up patches.

Bug: 212221722
Test: atest DisplayPolicyTests
Change-Id: Ib9e414627992b46b1a1d0afa314ea180dfdbc4d6