Expose IS_RUNNING_IN_TEST_HARNESS as static method.

This variable is now mutable, making the uppercase format misleading.
For instance, users might assume they can use this value in other
immutable properties, when they really should be accessing the latest
value every time they need it.

Context: https://source.android.com/docs/setup/contribute/code-style#follow-field-naming-conventions

Test: Manual
Bug: 271160958
Change-Id: Iaaa51d9153cb8a7d686c72e1210b1948029dcfd5