Fix density issues when maxUiWidth is set

Currently the code assumes that display size and density are
forced if and only if the mBase* values are different from
the mInitial* values. This is incorrect because base size
and density may not be forced but a result of applying
the maxUiWidth property.

Assuming the restricted density is forced makes
updateBaseDisplayMetricsIfNeeded() always pick it instead of
"newDensity". As result of that if we have maxUiWidth=1920 each
time we set a 4k mode the density will double as a result of the
calculations in updateBaseDisplayMetrics().

The same problem occurs with `adb shell wm size 3840x2160`,
because setForcedSize() assumes mBaseDensity should be used.
Instead this change picks a density which preserves the ratio
density/width constant.

Test: manually tested the abovementioned scenarios
Bug: 156100799
Change-Id: I8bda1b9883307fe94fed50be4a874b153e743a8f