Revert "Android: Increase default_min_sdk_version 19 -> 21"

This reverts commit ce14e58bbf6dcfcb74b3affccd9e45156657b9e7.

Reason for revert: The CL causes failures on Cronet KitKat builders https://ci.chromium.org/p/chromium/builders/ci/android-cronet-kitkat-arm-rel

The error code is INSTALL_FAILED_OLDER_SDK
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8878029920810653312/+/steps/cronet_smoketests_missing_native_library_instrumentation_apk_on_Android_device_Nexus_5/0/stdout

Original change's description:
> Android: Increase default_min_sdk_version 19 -> 21
> 
> Cronet is the only project that requires a lower minSdkVersion. To
> continue supporting building of their targets, this change enabled
> proguard on all of their apks in order to have all files desugared at
> the proper minApiVersion.
> 
> This change should speed up compiles somewhat, as it removes the need to
> generate main-dex-lists for legacy multidex purposes.
> 
> Bug: 1091919
> Change-Id: Ifd8089bbf4965b55a13dea11464ccda979fd5028
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2233581
> Reviewed-by: Peter Wen <wnwen@chromium.org>
> Commit-Queue: Andrew Grieve <agrieve@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#776166}

TBR=wnwen@chromium.org,agrieve@chromium.org

Change-Id: Id4b92d5549d13de9684f542ce646993c3833795f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1091919
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2236715
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776237}