Osu2: basic app setup

This defines MainActivity, which is the entry point
to the app.  All the resource files are copied from
the legacy Osu/ app, and might be updated in the future.

Bug: 62388032
Test: Manually start activity via adb
      "am start -n com.android.osu/.MainActivity"

Change-Id: I5b81fb8fc58fd94d5de2dbdb1e46b7ff6a40760b