Fix finetuning, add a finetuning screen, handle errors during importing model, update metadata format, add model exporting

Fix embedded tokenizer loading, implement new model management methods, implement model info loading, model importing

Since some variables with module LatinIME are defined but not used,
when compiled with build combination "sdk_pc_x86_64-userdebug" and
build command "mmm packages/inputmethods/LatinIME", the following
code lines will be reported that "variable 'XXX' set but not used".
(should be similar for all the other build combinations)

Repeated 10 times for each:
terminal_position_lookup_table.cpp:74:9 removedEntryCount
terminal_position_lookup_table.cpp:85:9 removedEntryCount
proximity_info_state_utils.cpp:493:9 tempTime
trie_map.cpp:56:9 unusedRegionSize
suggestion_results.cpp:100:9 index

Repeated 80+ times:
proximity_info_utils.h:75:25 proximityChar

With this patch we are removing some of the unused variables and
putting the C++ 17 attribute [[maybe_unused]] to the others which
are used for logging. Then all the related build warnings have been
eliminated.

Test: mmm packages/inputmethods/LatinIME, presubmit check.

Change-Id: Ia66766322d6ae8a010b1cb55cc22993fbc6d012c
Signed-off-by: Jing Mike <jingyangliu@eswincomputing.com>

See https://source.android.com/setup/contribute/respectful-code for reference

Bug: 161896447
Test: tapas LatinIME && m
Change-Id: Ic06f948c023a091e1532db826a78dd01031d7205

See https://source.android.com/setup/contribute/respectful-code for reference

Bug: 161896447
Change-Id: I171bc299e660f96f20041a67a3052bd8a2ed781c

Bug: 30413223
Test: make with WITH_TIDY=1 DEFAULT_GLOBAL_TIDY_CHECKS=-*,performance*
Change-Id: I06021a6597705515dc7a3cb81a7fbbac7ba344a1

Bug: 116614593
Test: build with WITH_TIDY=1
Change-Id: Ic2a04f82f36c2f7fe43109a7a74270044d227789

This CL addresses compiler warnings when building libjni_latinime.so
with NDK from Android Studio.

There should be no behavior change.

Bug: 110741422
Bug: 110757803
Test: tapas LatinIME LatinIMETests arm64 userdebug && make -j
Change-Id: Icc711dd46511a167b1fd90028552def5b2f9941f