Restore lazy loading of LoadedApk resources

This was broken in 8fff544ec349593cfa61ae2f5ec59419fff666b6. Since that change,
resources are always initialized in getPackageInfo(), even if they haven't been
loaded before (mResources == null).

It shouldn't be necessary in *any* case to preload the resources at this point.
Anything using the resources should call LoadedApk.getResources(), which will
initialize them just when needed. Setting mResources to null resets the cache
and will force a reload the next time the resources are requested.

Change-Id: I9bc9986297478e99752b1e2b9bd04dab3dcc9656
Themes: Support applying components from multiple themes [1/3]

The current implementation assumes the components being passed
into the ThemeService belong to the same theme.  This patch uses
a Map instead of a List to pass in a mapping of components to
package names, allowing us to apply several components from
different themes.

Requesting a theme change now takes a Map<String, String> which
maps components to the theme package name they will be applied
from.
Example:
    Map<String, String> componentMap = new HashMap<String,String>();
    componentMap.add(ThemesColumns.MODIFIES_ICONS, "com.mytheme.blue");
    componentMap.add(ThemesColumns.MODIFIES_OVERLAYS, "com.mytheme.red");

    ThemeManager tm = (ThemeManager) contex.getSystemService(Context.THEME_SERVICE);
    tm.requestThemeChange(componentMap);

Change-Id: I13fa17eabecb824884fe522d5dc1822a4d7c55f4