Fix long mac test times; only initialize host settings once

It looks like sometime in late February our Mac builds started taking
~10 minutes longer than before. On my local workstation the Soong tests
were taking >25 minutes before completing (likely because I don't have
the older SDKs installed, and we iterate from older to newer to find the
oldest installed SDK).

Most of this time was spend running the `xcrun` tools to interrogate the
system about which Mac SDKs are installed and where the tools are. This
will never change during any build or test, so wrap it all in a
sync.Once so that we only ever call them once.

And remove the macSdkPath variable, which has been unused for years and
no longer works (as we don't allow the use of xcode-select during the
build).

Bug: 153010389
Test: prebuilts/build-tools/build-prebuilts.sh on a Mac
Change-Id: I39b2d49739e628e4c11bec4805b25039115d2fd0
Merged-In: I39b2d49739e628e4c11bec4805b25039115d2fd0
(cherry picked from commit 6ba5367a)