[PA/MO] Allow ScopedSchedulerLoopQuarantine* utility use in early init

There are some cases we hit `PA_CHECK(ThreadCache::IsValid(tcache))`
because `ScopedSchedulerLoopQuarantineDisallowScanlessPurge` is used
before `ThreadCache::Init` call. This CL introduced initialization check
to support use of the utility before initialization.

Bug: 351974425
Change-Id: Ieb720754045d06928cfe128dcd40f8fc7b0467e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7532816
Commit-Queue: Mikihito Matsuura <mikt@google.com>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1581563}
NOKEYCHECK=True
GitOrigin-RevId: 7ad475ae91a7da1602985f2f789a5440540ddb73