Unset envsetup.sh variables inside build

Using ANDROID_BUILD_TOP and other envsetup.sh varialbes is already
forbidden in makefiles, but some tools run by the build (like
hidl-gen) will use it if it is set.  Since they are set by envsetup.sh,
and sourcing envsetup.sh before building is optional, unset them inside
the build to get consistent behavior.

Bug: 79250545
Test: m checkbuild
Change-Id: I3e7d9f01390ccdc34c49115f2f15cd7542d9974b