Allow tweaking jvm flags per mnemonic

[kisabaka]

The default Xms value for the JVM tool calls is too large and can cause builds to fail on machines with restricted resources (e.g building a large Android application a single Windows machine).

This change introduces a flag that allows tweaking JVM settings per action.

Example usage:

bazel build //my:application --config=android \
 --@rules_android//rules:mnemonic_jvm_flags=CompileAndroidResources=-Xms512M \
 --@rules_android//rules:mnemonic_jvm_flags=StarlarkMergeCompiledAndroidResources=-Xms1G

Relates to #302