gn2bp: fix arguments for jni_generator.py

Fix arguments for jni_generator.py:
--output_dir: gen/ replaced by $(genDir)
--input_dir: for source files only, drop ../../ and wrap in $(location <label>)

Test: m cronet_aml_base_base_jni_headers
Change-Id: Ie4868e47480789b599bc908402aa327ee75391fd