Enable DisplayList benchmarks to be built on Android

[gw280]

This allows the display_list_benchmarks target to be built on Android.

• Only initialise ICU in benchmarking::Main() when not on Android (it causes an abort on startup, and we don't use ICU for the DisplayList benchmarks)
• Don't build the software benchmarks on mobile platforms as we don't use software rendering and the suite takes a really long time to run
• Don't tie testing:opengl (TestGLSurface etc) to SwiftShader as we use them for benchmarking and we want to use the native/hw OpenGL driver for those

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[zanderso]

Does this work?

Suggested change

	if (is_mac || is_ios) {
	if ((is_mac && enable_unittests) || is_ios) {

[gw280]

That would work, but I think it makes more sense to expose the Metal utilities on all platforms that support Metal (iOS & Mac). If there isn't a target that depends on it, it won't get built anyway.