minui: add ARGB_8888 format

Minui currently really only supports composing in 2 different formats
(see gr_color()) with ALPHA always as MSB. However, some devices
interpret PixelFormat as either Big Endian (i.e. ARGB has alpha at MSB)
or Little Endian (i.e. BGRA has alpha at MSB).

This change attempts to give multiple options to specify the same format
depending on device interpretation, while keeping just 2 different
composition formats supported by minui.
* ARGB + BGRA: Pixels have (A)lpha at MSB and (B)lue at LSB
* RGBX + ABGR: Pixels have (A)lpha at MSB and (R)ed at LSB

With this in mind, limiting the use of png_set_bgr() to happen only for
(ARGB/BGRA) combination while leaving (RGBX/ABGR) unchanged.

Bug: 143480444
Test: Boot device with TARGET_RECOVERY_PIXEL_FORMAT := <<all>>
Change-Id: Ia0f94ccbc564b8def7c9416483712ff1abbbf49a