Add a TestApi to DM to get the last requested MPP

This CL exposes DisplayManager.getRequestedMinimalPostProcessing
for testing purposes. The call is propagated to
LogicalDisplay.getRequestedMinimalPostProcessingLocked().

Bug: 139369866

Test: m, flash, atest
CtsWindowManagerDeviceTestCases:MinimalPostProcessingTests

Change-Id: I7736d6e7a5c97d663111d10efa66d6d38a7e70bd