Merge "Import abseil to external/libtextclassifier" am: b90ac9e25c am: b16ff6fdc9 am: 681103d7a0

Original change: https://android-review.googlesource.com/c/platform/external/libtextclassifier/+/1477096

Change-Id: I71b2ff63d725eed44ff037202beb6ac78ce03d7d