Issue title: Android Support for API < 24
 Issue body: I've a problem with this great library with Android API < 24 (for 25+ the lib works well).

- IDE: Android Studio
- Gradle: 4.1.3

Following the [Sample project](https://github.com/andob/OpenPDFAndroidSample) I've added the dependencies:
`implementation 'com.github.librepdf:openpdf:1.3.24'
implementation 'ro.andob.androidawt:androidawt:1.0.4'`
The compile option is set to Java 1.8
and Maven repo:
`maven { url "https://maven.andob.info/repository/open_source" }`
I've noticed that your example project has min API set to 16, mine is 21.

The stacktrace error is:
`java.lang.NoSuchMethodError: No virtual method chars()Ljava/util/stream/IntStream; in class Ljava/lang/String; or its super classes (declaration of 'java.lang.String' appears in /system/framework/core-libart.jar)`

and code reference is:
`document.open();`
method.

I've tried to add the following dependency:
`coreLibraryDesugaring 'com.android.tools:desugar_jdk_libs:1.1.5'`
and
`// Flag to enable support for the new language APIs
    coreLibraryDesugaringEnabled true`

as described in [Android Devs](https://developer.android.com/studio/write/java8-support#library-desugaring) post, but didn't work.

Can anyone help me to solve this issue?

Thanks for your support.

 Comments: 
 Comment 0: Pull request welcome.
 Comment 1: Currently I've solved downgrading the dependency
`implementation 'com.github.librepdf:openpdf:1.0.5'
implementation 'ro.andob.androidawt:androidawt:1.0.4'`

As far as I've a little chunk of time I'll try the PR.

Thanks
 Comment 2: There is a java7 Branch, which you can take a look. The main Problem of older APIs for Android is that there is no support for Streams and Lambdas. The branch is quite updated, so any "upgrades" there are also welcome.
 Comment 3: I've noticed the problem of Lambdas and Streams. 
My idea is to add 
`coreLibraryDesugaring 'com.android.tools:desugar_jdk_libs:1.1.5'` dependency
and
`coreLibraryDesugaringEnabled true`
to allow Lambdas and Streams usage in API <24.
{'source': 'GithubIssues', 'repo_name_with_owner': 'LibrePDF/OpenPDF', 'issue_id': 892082803, 'repo_stars': 2468.0}