Add AndroidClock which overrides nanoTime with `SystemClock.elaps…

[atulgpt]

…edRealtimeNanos()`

Fixes #1364

But doesn't make it overridable

[fractalwrench]

I think we can improve this using the suggestions stated here: #1364 (comment)

[atulgpt]

Hi @fractalwrench the definition of nanoTime is as below(taken from io.opentelemetry.sdk.common.Clock)

  /**
   * Returns a time measurement with nanosecond precision that can only be used to calculate elapsed
   * time.
   */

so this API is only suitable for calculating the elapsed time. So do we need to do what you are suggesting?

[fractalwrench]

nanoTime does not guarantee any correlation to wall-clock time. OTLP requires a timestamp - i.e. elapsed nanoseconds since the Unix epoch.

[atulgpt]

OTLP requires a [timestamp](https://opentelemetry.io/docs/specs/otel/logs/data-model/#field-timestamp) - i.e. elapsed nanoseconds since the Unix epoch. but for this there are other functions in io.opentelemetry.sdk.common.Clock which are now and now(boolean highPrecision) both returns timestamp in nano since epoch.

That's why I was asking do we want to change the behavior of the third method nanoTime which is documented to use only for elapsed time?