Fix documentation generation with OpenJDK 9 javadoc.

"make docs" breaks ("lint: @attr must be a field") on three @attr ref
statements in LinearLayoutManager. These are the only @attr ref
statements that reference the support library's resources from
framework (the others are within framework or within the support
library); it may be that the R class that they're referencing is
merely in the wrong package, but changing it to com.android.internal.R
or android.R didn't fix the issue.

Since LinearLayoutManager is an internal class, it's not worth the
effort of trying to make this work. This CL drops the three offending
@attr ref statements, thereby fixing the build.

Bug: 62049770
Test: Treehugger

Change-Id: Ife01201019f4fffd5552da11591661539ca40fdd