Build egl_display extension string using temporary vector

Currently it is very easy to forget to add a trailing space when adding to the extension string. This has already caused a major bug fixed by 0bc64a80.

Build the string using a vector instead similar to libANGLE::Context::initExtensionStrings() does it. This is much less error prone than the current code.

Test: Added unit test
Test: Ran atest EGL_test
Test: Confirmed string output is the same with print
Bug: 285606242
Change-Id: I7f837a3009f0ccfedcf44f3d7ff709a269d0034f