Make it easier to enable obfuscation and optimization.

With this change, use:
* LOCAL_PROGUARD_ENABLED := obfuscation  # to enable obfuscation
* LOCAL_PROGUARD_ENABLED := optimization # to enable optimization
* LOCAL_PROGUARD_ENABLED := obfuscation optimization # to enable both

Now the meaning of the LOCAL_PROGUARD_ENABLED options:
* full:
    Use the build system's default configurations:
    with shrink but no obfuscation or optimization,
    global proguard flags in build/core/proguard.flags
    are applied.
* custom:
    The same as "full" except no aapt-generated resource-related
    proguard flags.
* nosystem:
    Don't use any build system's default configurations; but
    aapt-generated proguard flags are still applied. You are
    responsible for any other flags.
* disabled:
    Disable proguard.
* obfuscation:
    The same as "full" but with obfuscation enabled.
* optimization:
    The same as "full" but with optimization enabled.
* no value (the default):
    The build system chooses the proper value: "full" if it's an
    app; "disabled" if it's a library.

You can use more than 1 of them in a meaningful combination,
for example:
LOCAL_PROGUARD_ENABLED := obfuscation optimization

Bug: 10307372
Change-Id: Id248caca3048e99547f16559fae74f4afe85c354