Generic Mutable class for HIDL code.

In general, using Mutable inside of structures
or as API arguments is a code smell. However, it
is the most syntactically clean way of fetching
something out of a lambda or inner class.

Using this, one can do:
Mutable<Result> res = new Mutable<>();
hidlObject.foo((...) -> {
    res.value = ...;
});

There is an alternative:
Result a[] = new Result[1];
hidlObject.foo((...) -> {
    a[0] = ...;
});

However, this alternative syntax is relatively messy.

Bug: N/A
Test: boot, use w/ wifi
Change-Id: Ibff13c653cc17bd25ddbb0534ba21ef485bff7aa