Initialize variable to fix wrong odex installed

When "PRODUCT_DEX_PREOPT_GENERATE_DM_FILES := true" and
"PRODUCT_DEX_PREOPT_DEFAULT_COMPILER_FILTER := verify" are set,
boot jar may wrongly refer to previous module's variables such as
my_generate_dm, my_built_dm and my_installed_dm. And then cause
unexpected files installed along with this boot jar.

So initialize my_generate_dm to empty at first.

Bug: 117526952
Test: make out/target/product/generic/module-info.json
Test: cat out/target/product/generic/module-info.json | grep '"apache-xml":'
Change-Id: Iaa851f4fb530b3446de3dbcb1f54e545938830ce