Separate source comment URLs from real data with <eat-comment>

We're adding in XML comments into the output files from the resource
merger which point back to the real origin of the merged items.

However, aapt will interpret these comments as user comments for the
resources, and these are converted into Javadocs in the R class,
meaning that the paths show up as documentation.

Aapt has a special <eat-comment> tag for this specific purpose, which
causes the accumulated comments to be discarded. This changeset makes
the resource merger emit these elements after the source URL comments.

It also tweaks a user message to make it clear to the user how to
continue (when the incremental merger fails), and exposes some APIs
used by the lint integration in Gradle (from package protected to
public.)

Change-Id: If44e65c54efab1fc32762a89dc1badecdff582ee

Conflicts:

	common/src/main/java/com/android/SdkConstants.java
	lint/cli/src/main/java/com/android/tools/lint/LintCliClient.java
	lint/cli/src/main/java/com/android/tools/lint/Reporter.java
	sdk-common/src/main/java/com/android/ide/common/res2/MergedResourceWriter.java
	sdk-common/src/main/java/com/android/ide/common/res2/ResourceSet.java
	sdk-common/src/test/java/com/android/ide/common/res2/ResourceMergerTest.java