Allow boards to turn overriding commands into an error

Many boards have warnings like this, saying that we defined a build
rule, but later something else came in and overrode it with something
else:

art/build/Android.gtest.mk:677: warning: overriding commands for target `test-art-target-gtest-cmdline_parser_test'
art/build/Android.gtest.mk:674: warning: ignoring old commands for target `test-art-target-gtest-cmdline_parser_test'

Beyond the obvious problem of replacing the rule with something else,
target-specific variables can be combined as well, leading to some very
strange problems.

Since so many boards still have problems like this, but we don't
currently have any global problems, add a flag so that we can mark
boards as not broken. This should prevent regressions while we clean up
the individual problems.

Once the non-broken devices number significantly more than the broken
devices, we'll switch this default. And once they're all cleaned up this
variable will become obsolete, and these warnings will always be errors.

Bug: 77611511
Test: lunch aosp_arm-eng; m nothing
Test: lunch aosp_marlin-eng; m nothing
Test: build_test on all downstream branches
Change-Id: I43b645658f468c23a5b9ebcfcd9d4516537db540