Fix DPI handling after hwcomposer 2.4 update aosp/1843498 accidently dropped the factor of 1000 that was applied in Display::init(). Instead, move the factor of 1000 to DisplayConfig and document the factor better. Bug: b/201578179 Bug: b/202358218 Test: `launch_cvd` and inspect DPI Change-Id: I40fb96900ea864497eee093049b8b707e6991298 Merged-In: I40fb96900ea864497eee093049b8b707e6991298