Move code to plugin core lib

This contains the core interfaces for plugin support, e.g. Plugin.java
and the associated annotations. It is reused by the shared library and
plugin interfaces in both sysui and launcher are built off of it.

Test: atest com.android.systemui.shared.plugins
Test: cd $ANDROID_BUILD_TOP/frameworks/base/packages/SystemUI/plugin_core \
    && mma -j32 && cd -
out/target/product/$TARGET_PRODUCT/obj/JAVA_LIBRARIES/PluginCoreLib_intermediates/javalib.jar
contains the created jar and is useable in launcher

Bug: 115877296
Change-Id: I82ca7398e882d6432333238e2f77a12e776f1d76