Add condition check of KEEP_APEX_INHERIT

Add environment variable KEEP_APEX_INHERIT condition check to set the
proper value of Soong conditional variable library_linking_strategy. If
the KEEP_APEX_INHERIT exists, the "min_sdk_version" in condition will be
set to "apex_inherit". For more detail:
https://docs.google.com/document/d/1R2vZw0cQa-haAMgFyQ682uSq9aGBNQrzMHKIsU17-XY/edit?usp=sharing&resourcekey=0-gUbs463r9LCKs7vdP_Xkmg

Test: build module locally, and check the corresponding change.
Presbumit
Bug: 254634795

Change-Id: I0ddc999a19d9f164477d69be0e4aebc91e06a3ad
Merged-In: Ic5da1c83b3b9c090bc2a24d36b1a4ac936ce1a59