AAPT2: Fix raw string parsing

Trim whitespace of raw strings when parsing xml values. This change
trims the whitespace of style items. For example:

<item name="viewInflaterClass">com.helloworld.Inflater
</item>

This will be trimmed to not include a trailing whitespace.

Bug: 109666819
Test: aapt2_tests
Change-Id: I0c1fbb3abdc7e609316c92e59ccaf0573b07e5a7