[Android] Add the method 'Log.getStackTraceString'

[ColdPaleLight]

fix flutter/flutter#110856

I found that Log.getStackTraceString is not exposed. I think it's very helpful to be able to get stack trace string from Throwable or Exception.

In fact, there are many codes in flutter that need to use this capability. In the current implementation, each place is implemented separately.

This PR proposes to expose Log.getStackTraceString to simplify the codes and for similar scenarios in the future.

c.f.
https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/util/Log.java#371

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[ColdPaleLight]

(Test added)