Allow GenericDocument accept a String property longer than 20_000.

This limit was removed from the AppSearch Jetpack API meaning that
a client could construct a GenericDocument successfully, but then
have a PutDocuments call fail when converting from
androidx.appsearch.GenericDocument
to android.appsearch.GenericDocument.

Icing still has a total document size limit which is 16 MiB.
The put call will fail if it trying to write such document.


Bug: 192909904
Test: GenericDocumentTest
Change-Id: I86f97acc3a8e0f1c25fadf597aed9f42a2c493eb