Add support for ANDROID_RUNTIME_ROOT variable

This commit introduces the ANDROID_RUNTIME_ROOT environment
variable in preparation for its use within the runtime module.

As a module, ART must no longer use the Android-wide
ANDROID_ROOT environment variable as the base of default
paths for module content. ANDROID_RUNTIME_ROOT will take over
the responsibility. It remains a variable to unify host
and device treatment.

Some test methods (SetUpAndroidData, TearDownAndroidData
SetUpAndroidRoot) have been renamed to clarify their purpose.

Bug: 119935277
Test: build / treehugger
Change-Id: I7c7de52911cbfa56cef90270425ed369176c4767