Add VMRuntime.isJavaDebuggable().

This is needed by ART tests to find out if the runtime is running in java
debuggable mode.

Bug: 79914966
Test: $ art/test.py -b --debuggable --host -t 674-hiddenapi

(cherry picked from commit ba1992db)

Merged-In: I93279bcb9b73d60dbee386f33f8821132bd74a51
Change-Id: Iede6c8df27783c23683d6522055534f221c1cede