AAPT2: Add flag to compile command for outputting symbols

Only XML files can define resources inside of them, so the fragment
R.txt will only be created for XML files. The fragment R.txt will
contain files defined inside the XML files and the file itself.
For example for res/layout/my_layout.xml that defines "@+id/myView" the
fragment R.txt will contain "default int id myView" and "default int
layout my_layout".
Resources defined with the "public" keyword will have the word "public"
in the partial R.txt, resources defined with the "java-symbol" keyword
will have the word "private, and all other resources will have the word
"default".
If a string is declared in values/strings.xml as:
'<string name="foo">text</string>'
then the partial R.txt will contain "default int string foo". If the
same string is also marked as public in the values/public.xml as:
'<public type="string" name="foo" id="0x7f000001"/>
then the partial R.txt for that file will cointain:
"public int string foo".
Also, the resource IDs will be skipped as this is only for compilation,
proper IDs will be generated at linking phase.

Test: manual
Change-Id: I37d07d5ee4a9f2e5a60a54e48579eba86ae7dd60