Prevent private VDs from being density scaled.

Automotive has a compatibility layer that automatically scales the
density of non-automotive applications including CTS. However this
scaling shouldn't happen on private virtual displays created by apps
themselves who may pass an explicit density value into the virtual
display constructor.

Bug: 436907259
Test: atest CtsInputTestCases && atest
android.app.servertransaction.TransactionParcelTests && atest
com.android.server.wm.ActivityRecordTest && atest
android.app.activity.ActivityThreadTest
Flag: EXEMPT bug:_fix

Change-Id: I56be702bbdf96cbb8cdb5941edccc632b7a8806e