[generator] Inherit declaring type's 'deprecated-since' if lower than member's.

[jpobst]

Imagine a type is deprecated in API-25 but its members are not explicitly marked as deprecated:

@Deprecated
public class TabActivity extends ActivityGroup {
  public void setDefaultTab(String tag) { ... }
}

Then in API-29 the member is explicitly marked as deprecated:

@Deprecated
public class TabActivity extends ActivityGroup {
  @Deprecated
  public void setDefaultTab(String tag) { ... }
}

Due to the way api-merge calculates deprecated-since, these would end up with different values, and the resulting C# API would look like:

[ObsoletedOSPlatform("android25.0")]
public class TabActivity : ActivityGroup
{
  [ObsoletedOSPlatform("android29.0")]
  public void SetDefaultTab (string tag) { ... }
}

While technically "fine", it is confusing that the method says it was obsoleted in API-29 instead of API-25.

To fix this, if a member has a "higher" deprecated-since than its declaring type, we set the member to the declaring type's value.