refactor: modernize --res-resolve-mode and dummies

[IgorEisberg]

• Since --res-resolve-mode has a new name in 3.x we can afford to update the modes to better describe their meaning.

• default: Adds dummy entries only for resources that cannot be resolved.
  (this is done after ARSC parsing, during resolution and res file decoding stages)
• greedy: Adds dummy entries for all missing resources in the resource table.
  (this is done during ARSC parsing, at the end of each package)
• lazy: Ignores resources that are missing or cannot be resolved.

(Docs will be updated after merge.)

• APKTOOL_MISSING_ is now also APKTOOL_DUMMY_ (they are the same thing, just were added at different stages).

• Improve dummy serialization based on each item/bag type's default value.

Case study:

<resources>
    <anim name="DUMMY" />
    <animator name="DUMMY" />
    <array name="DUMMY" />
    <attr name="DUMMY" />
    <bool name="DUMMY" />
    <color name="DUMMY" />
    <dimen name="DUMMY" />
    <drawable name="DUMMY" />
    <font name="DUMMY" />
    <fraction name="DUMMY" />
    <id name="DUMMY" />
    <integer name="DUMMY" />
    <interpolator name="DUMMY" />
    <layout name="DUMMY" />
    <menu name="DUMMY" />
    <mipmap name="DUMMY" />
    <plurals name="DUMMY" />
    <raw name="DUMMY" />
    <string name="DUMMY" />
    <style name="DUMMY" />
    <transition name="DUMMY" />
    <xml name="DUMMY" />
</resources>

aapt2 dump resources:

type anim id=01 entryCount=1
  resource 0x7f010000 anim/DUMMY
    () @null
type animator id=02 entryCount=1
  resource 0x7f020000 animator/DUMMY
    () @null
type array id=03 entryCount=1
  resource 0x7f030000 array/DUMMY
    () (array) size=0
      []
type attr id=04 entryCount=1
  resource 0x7f040000 attr/DUMMY
    () (attr) type=any
type bool id=05 entryCount=1
  resource 0x7f050000 bool/DUMMY
    () @null
type color id=06 entryCount=1
  resource 0x7f060000 color/DUMMY
    () @null
type dimen id=07 entryCount=1
  resource 0x7f070000 dimen/DUMMY
    () @null
type drawable id=08 entryCount=1
  resource 0x7f080000 drawable/DUMMY
    () @null
type font id=09 entryCount=1
  resource 0x7f090000 font/DUMMY
    () @null
type fraction id=0a entryCount=1
  resource 0x7f0a0000 fraction/DUMMY
    () @null
type id id=0b entryCount=1
  resource 0x7f0b0000 id/DUMMY
    () (id)
type integer id=0c entryCount=1
  resource 0x7f0c0000 integer/DUMMY
    () @null
type interpolator id=0d entryCount=1
  resource 0x7f0d0000 interpolator/DUMMY
    () @null
type layout id=0e entryCount=1
  resource 0x7f0e0000 layout/DUMMY
    () @null
type menu id=0f entryCount=1
  resource 0x7f0f0000 menu/DUMMY
    () @null
type mipmap id=10 entryCount=1
  resource 0x7f100000 mipmap/DUMMY
    () @null
type plurals id=11 entryCount=1
  resource 0x7f110000 plurals/DUMMY
    () (plurals) size=0
type raw id=12 entryCount=1
  resource 0x7f120000 raw/DUMMY
    () @null
type string id=13 entryCount=1
  resource 0x7f130000 string/DUMMY
    () ""
type style id=14 entryCount=1
  resource 0x7f140000 style/DUMMY
    () (style) size=0
type transition id=15 entryCount=1
  resource 0x7f150000 transition/DUMMY
    () @null
type xml id=16 entryCount=1
  resource 0x7f160000 xml/DUMMY
    () @null

We learn that:

• default value for bag types (array, attr, plurals, style) is an empty bag of the given type.
• default value for id type is a special placeholder (we already handled that).
• default value for string type is the empty string ("").
• default value for the rest of the item types is @null.

Use that to serialize them accordingly and add dummies with the appropriate default value.
The result is avoiding unnecessary <item> tags and unnecessary @nulls where empty tags mean the same.
Much cleaner output, <item> tags are used only when it's unavoidable.

Also added a few test cases for this.