minui: Clean up the use of `rotation`.

This CL uses scoped enum for GRRotation. It also refactors the previous
comparisions to improve readability -- it was hard to tell the meaning
of `if (rotation % 2)` unless by checking the exact values.

It also temporarily adds STRINGFY to expand the macro of
DEFAULT_ROTATION. This will be removed once minui is changed from
reading build time vars to runtime properties.

Test: Build and boot into recovery image on marlin.
Test: Set `TARGET_RECOVERY_DEFAULT_ROTATION := ROTATION_LEFT` and check
      the display under recovery with `Run graphics test`.
Change-Id: Ieb08b1cd13c284f8dcf906cb2a1430c6bae2835b