multiplatform: split common/android/desktop (#2672)

* multiplatform: relocated code to its new place

* code becomes better

* renamed file

* fixes for BASE64 and images, and changes for appFileUri

* different Base64 for both platforms

* fix file saving on long click

* platformCallbacks refactoring

* renamed callbacks to platform

* eol

---------

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

apps/multiplatform/common/src/androidMain/res/drawable/edit_text_cursor.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="rectangle" >
+  <solid android:color="@color/highOrLowLight" />
+  <size android:width="1dp" />
+</shape>