Do not include any documentation when building stubs

We only need documentation (javadocs) when building docs stubs,
for regular stubs, we can drop printing out all the docs.

Tested with:
touch frameworks/base/core/java/android/view/View.java && time make -j framework

Before (with just r.android.com/1254909)
Run #1 2m4.779s
Run #2 2m6.672s
Run #3 2m4.179s

Average: 125.2 seconds

After (with this patch and r.android.com/1254909)
Run #1 1m58.682s
Run #2 1m58.820s
Run #3 1m57.724s

Average: 118.4 seconds

Speedup ~6.8 seconds (5.4%)

Bug: 151160048
Test: make checkapi
Change-Id: I27eafb5a61bb9d26a74ca25f84c9daca17a8394f

java/droiddoc.go

@@ -1345,6 +1345,7 @@ func (d *Droidstubs) stubsFlags(ctx android.ModuleContext, cmd *android.RuleBuil
 		cmd.FlagWithArg("--doc-stubs ", stubsDir.String())
 	} else {
 		cmd.FlagWithArg("--stubs ", stubsDir.String())
+		cmd.Flag("--exclude-documentation-from-stubs")
 	}
 }