Raise target mobile device DPI

A target mobile DPI of 135 is rather low and results in excessively
large scale factors, so everything is too big.. Let's raise it a
little bit to improve the default level of information density on
these devices that are designed to be held close to the face.

This has the effect of changing the calculated default scale factor for
the following devices:

| Device     | Device DPI | Old scale factor | New scale factor |
| ---------- | ---------- | ---------------- | ---------------- |
| Steam Deck | 204        | 1.5              | 1.25             |
| PinePhone  | 268        | 2.0              | 1.75             |
| OnePlus    | 388        | 3.0              | 2.5              |

Devices are taken from the autotest data found at
https://invent.kde.org/plasma/kscreen/-/blob/master/tests/kded/configs/AutogeneratedMobileScreenScales.json

src/outputconfigurationstore.cpp

@@ -519,7 +519,7 @@ double OutputConfigurationStore::targetDpi(Output *output) const
             return 125;
         } else {
             // phone screens: even closer than laptops
-            return 136;
+            return 150;
         }
     } else {
         // "normal" 1x scale desktop monitor dpi