Use OkHTTP to implement HTTPContext on Android

[ljbade]

Closes #823

[ljbade]

This is a pretty big merge with lots of new JNI code.

@kkaefer @jfirebaugh Do you mind giving this a review?

[jfirebaugh]

I'm not really familiar with any of this code. @incanus or @bleege are probably better reviewers.

[mb12]

1.) Can you also share improvements in APK file size and any changes in performance w.r.t runtime and memory?
2.) Also do tile requests for Android do a lookup in sqlite cache before going over the network? Will this also be moved to managed code?

[ljbade]

@mb12 In this branch everything still goes via sqlite cache. Can't see why we can't use Java side caching.

Does OkHTTP have support for that?

I haven't done checks of the APK size or performance comparisons yet.

[ljbade]

Going to collect here before/after .aar file sizes:

For just ARM 32bit lib:

• Before: 3.0MB
• After: 2.0MB

All libs:

• Before:
• After:

[ljbade]

@kkaefer Do you know a good way to quickly benchmark the tile fetch performance with these changes?