Support customizing toolchain level jvm_flags

Users are unable to easily customize the jvm_flags backing some of the tools in [toolchain.bzl](https://github.com/bazelbuild/rules_android/blob/ed557a4948b80eb27a6a8da3bd1302b85c506a0d/toolchains/android/toolchain.bzl#L18) as they are all being passed as their raw deploy jars instead of their executable Bazel types.

This means that JVM resource tuning changes like https://github.com/bazelbuild/rules_android/pull/140 will always need to be checked into `rules_android` or kept in forks rather than leveraging the existing attributes that Bazel provides.