Don't use XrHostActivity during tests

Adds infrastructure to tell XrSessionCoordinator whether an additional
activity (the XrHostActivity) should be created or not. At present, this
will only be true when the XrServiceTestHook is set (i.e. during tests).
While attempting to enable the current WebXr xr_browser_tests in
android_browsertests issues were discovered with the activity lifecycle.

Issue 381076469 tracks a fix to hopefully remove this workaround.

Bug: 381000093,381076469
Change-Id: I16564f23ead58d7e4f31af1d0f0c859a05a0d566
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6065324
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1391412}