android package with libc++_shared.so

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> - if this is a _bug_ or _feature request_, fill the form below as best as you can.

### Description of the problem / feature request:

On Android, bazel always libc++_static instead of libc++_shared.so

I want to make so link with libc++_shared.so, but bazel always libc++_static.a.

https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/android/AndroidBinary.java

```
private static RuleConfiguredTargetBuilder init(
RuleContext ruleContext,
NestedSetBuilder filesBuilder,
CppSemantics cppSemantics,
JavaSemantics javaSemantics,
AndroidSemantics androidSemantics)
throws InterruptedException, RuleErrorException {

ResourceDependencies resourceDeps =
ResourceDependencies.fromRuleDeps(ruleContext, /* neverlink= */ false);

validateRuleContext(ruleContext);

NativeLibs nativeLibs =
NativeLibs.fromLinkedNativeDeps(
ruleContext,
ImmutableList.of("deps"),
androidSemantics.getNativeDepsFileName(),
cppSemantics); // always create static mode cc_library!!!!!!
```

### Feature requests: what underlying problem are you trying to solve with this feature?

I have many libraries depends libc++, if I use libc++_static, every library contains libc++_static, which make apk too large.

I want to package libc++_shared.so, how do I do?

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

bazel build a android apk.

### What operating system are you running Bazel on?

mac 10.14.6

### What's the output of `bazel info release`?

> Replace this line with your answer.

### If `bazel info release` returns "development version" or "(@non-git)", tell us how you built Bazel.

0.29.1-homebrew

### What's the output of `git remote get-url origin ; git rev-parse master ; git rev-parse HEAD`

### Have you found anything relevant by searching the web?

no found.

### Any other information, logs, or outputs that you want to share?

no