Retain parsed attribute type

If the value of an attribute enum is defined as a hexadecimal integer,
flatten uses of the attribute as with the
android::Res_value::TYPE_INT_HEX type.

This change adds a "type" field to pb::Attribute::Symbol, which if left
unset, will have a default value of android::Res_value::TYPE_INT_DEC
when deserialized by aapt2.

Bug: 124474141
Test: aapt2_tests and manual compilation of files and inspection using
`aapt2 dump chunks`
Change-Id: Ibf12394284fdbe3a8047f7ecf4fe68517dfc3abb