Refactor ANGLE Platform support

Migrate almost all angle-platform specific code out of egl_display.cpp
to egl_angle_platform.cpp. This isolates use of angle "Platform.h" code
to only egl_angle_platform.cpp. I also removed the class wrapper for
static platform functions as it added no benefit.

Also syncing up latest Platform.h from ANGLE.

Test: build and flash device, run app w/ ANGLE enabled, systrace
Change-Id: I257deb28ee41aa6b3258a373aaa481f71968315f