Speed up ScopedPrimitiveArrayRO

For small arrays <= 1024 elements, we use GetArrayRegion with stack
allocated backing storage instead of GetArrayElements. The speedup
comes from the fact that GetArrayRegion doesn't need to allocate a
copy of array.

Timing results for:
https://android-review.googlesource.com/#/c/169727/

BEFORE

Host:
Byte length=1 ns/op=452.2633712
Short length=1 ns/op=468.6469192
Int length=1 ns/op=422.5663486
Long length=1 ns/op=423.4946638
Byte length=8 ns/op=424.8546962
Short length=8 ns/op=424.3842626
Int length=8 ns/op=424.3816324
Long length=8 ns/op=452.9407552
Byte length=64 ns/op=441.674149
Short length=64 ns/op=472.2310028
Int length=64 ns/op=455.6142958
Long length=64 ns/op=487.6810328
Byte length=512 ns/op=454.0858634
Short length=512 ns/op=497.675829
Int length=512 ns/op=481.2103832
Long length=512 ns/op=507.4674306
Byte length=4096 ns/op=531.7923602
Short length=4096 ns/op=648.254466
Int length=4096 ns/op=578.0794842
Long length=4096 ns/op=563.3641424

N5:
Byte length=1 ns/op=3118.2292802
Short length=1 ns/op=3083.7797488
Int length=1 ns/op=3177.8996446
Long length=1 ns/op=3151.0892072
Byte length=8 ns/op=3287.7326966
Short length=8 ns/op=3126.5799676
Int length=8 ns/op=3197.3203112
Long length=8 ns/op=3201.0613218
Byte length=64 ns/op=3178.7730612
Short length=64 ns/op=3162.2255092
Int length=64 ns/op=3241.3191864
Long length=64 ns/op=3117.6674886
Byte length=512 ns/op=3106.540978
Short length=512 ns/op=3183.0501344
Int length=512 ns/op=3513.8142382
Long length=512 ns/op=3592.4372798
Byte length=4096 ns/op=3552.9428216
Short length=4096 ns/op=3997.586686
Int length=4096 ns/op=4545.2151442
Long length=4096 ns/op=4371.4996338

AFTER

Host:
Byte length=1 ns/op=261.764995
Short length=1 ns/op=238.6584262
Int length=1 ns/op=262.856902
Long length=1 ns/op=238.0286378
Byte length=8 ns/op=242.4962264
Short length=8 ns/op=247.8668746
Int length=8 ns/op=263.9420024
Long length=8 ns/op=243.3675294
Byte length=64 ns/op=243.3372902
Short length=64 ns/op=249.0408034
Int length=64 ns/op=250.2168296
Long length=64 ns/op=253.837053
Byte length=512 ns/op=252.8071526
Short length=512 ns/op=271.689653
Int length=512 ns/op=291.8933238
Long length=512 ns/op=341.6303746
Byte length=4096 ns/op=540.8749038
Short length=4096 ns/op=661.7068158
Int length=4096 ns/op=569.030217
Long length=4096 ns/op=565.0571702

N5:
Byte length=1 ns/op=1209.4042078
Short length=1 ns/op=1207.2498746
Int length=1 ns/op=1377.0557706
Long length=1 ns/op=1348.6359368
Byte length=8 ns/op=1260.0724162
Short length=8 ns/op=1229.6107912
Int length=8 ns/op=1414.1408952
Long length=8 ns/op=1394.1158746
Byte length=64 ns/op=1232.532947
Short length=64 ns/op=1228.1248122
Int length=64 ns/op=1468.2667912
Long length=64 ns/op=1404.232687
Byte length=512 ns/op=1256.976093
Short length=512 ns/op=1335.5540308
Int length=512 ns/op=1599.6927076
Long length=512 ns/op=1769.8866764
Byte length=4096 ns/op=3310.2486656
Short length=4096 ns/op=3956.6322484
Int length=4096 ns/op=4374.6221234
Long length=4096 ns/op=4256.71598

Bug: 19664826
Change-Id: I703d7346de732199be1feadbead021c6647a554a