Fix core_settings.gradle to not assume exoplayerRoot is absolute

Gradle warns against passing a relative path to `new File(String)`:
https://docs.gradle.org/current/userguide/working_with_files.html#sec:single_file_paths

This change fixes all usages of `exoplayerRoot` to pass it to Gradle's
`Project.file()` first, which returns an absolute `File`.

To reproduce the problem in Issue: #8927:
1. Checkout ExoPlayer git project, to e.g. `~/ExoPlayer/exoplayer-git`
2. Create a new Android Studio project in e.g. `~/AndroidStudioProjects/exoplayer-test`
3. Edit the new project's `settings.gradle` file as described in
   https://github.com/google/ExoPlayer/blob/release-v2/README.md
   using a relative path for `exoplayerRoot`:
   ```
   gradle.ext.exoplayerRoot = '../../ExoPlayer/exoplayer-git'
   ```
4. In a shell:
   ```bash
   $ cd ~/AndroidStudioProjects/exoplayer-test/app
   $ ../gradlew build
   ```

(Step 4 is important, it seems running `./gradlew` from the project root
doesn't trigger the relative path problem)

This change fixes the problem, and also works with `exoplayerRoot` as a
`File` or `Path` object. `String`, `File` and `Path` all work with relative or
absolute paths:
```
gradle.ext.exoplayerRoot = '/home/ibaker/ExoPlayer/exoplayer-git'
gradle.ext.exoplayerRoot = '../../ExoPlayer/exoplayer-git'
gradle.ext.exoplayerRoot = new File('/home/ibaker/ExoPlayer/exoplayer-git')
gradle.ext.exoplayerRoot = new File('../../ExoPlayer/exoplayer-git')
gradle.ext.exoplayerRoot = Paths.get('/home/ibaker/ExoPlayer/exoplayer-git')
gradle.ext.exoplayerRoot = Paths.get('../../ExoPlayer/exoplayer-git')
```

Note: The Path versions above require importing `java.nio.file.Paths`
and changing the `apply from:` line in the project's settings.gradle
file to something like:
```
apply from: file(gradle.ext.exoplayerRoot.resolve('core_settings.gradle'))
```
It's assumed that a project wanting to pass a `Path` will make these
changes.

Issue: #8927
PiperOrigin-RevId: 374421627

README.md

@@ -104,12 +104,12 @@ git checkout release-v2
 ```
 
 Next, add the following to your project's `settings.gradle` file, replacing
-`/absolute/path/to/exoplayer` with the absolute path to your local copy:
+`path/to/exoplayer` with the path to your local copy:
 
 ```gradle
-gradle.ext.exoplayerRoot = '/absolute/path/to/exoplayer'
+gradle.ext.exoplayerRoot = 'path/to/exoplayer'
 gradle.ext.exoplayerModulePrefix = 'exoplayer-'
-apply from: new File(gradle.ext.exoplayerRoot, 'core_settings.gradle')
+apply from: file("$gradle.ext.exoplayerRoot/core_settings.gradle")
 ```
 
 You should now see the ExoPlayer modules appear as part of your project. You can

RELEASENOTES.md

@@ -2,6 +2,10 @@
 
 ### dev-v2 (not yet released)
 
+*   Core Library:
+    *   Fix gradle config to allow specifying a relative path for
+        `exoplayerRoot` when [depending on ExoPlayer locally](README.md#locally)
+        ([#8927](https://github.com/google/ExoPlayer/issues/8927)).
 *   Extractors:
     *   Add support for MPEG-H 3D Audio in MP4 extractors
         ([#8860](https://github.com/google/ExoPlayer/pull/8860)).
@@ -49,7 +53,8 @@
     *   Use the PRECISE attribute in EXT-X-START to select the default start
         position.
 *   PlayerNotificationManager:
-    *   Add `PendingIntent.FLAG_IMMUTABLE` flag to BroadcastReceiver to support Android 12.
+    *   Add `PendingIntent.FLAG_IMMUTABLE` flag to BroadcastReceiver to support
+        Android 12.
 
 ### 2.14.0 (2021-05-13)
 

core_settings.gradle

@@ -11,10 +11,9 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-def rootDir = gradle.ext.exoplayerRoot
+def rootDir = file(gradle.ext.exoplayerRoot)
 if (!gradle.ext.has('exoplayerSettingsDir')) {
-    gradle.ext.exoplayerSettingsDir =
-            new File(rootDir.toString()).getCanonicalPath()
+    gradle.ext.exoplayerSettingsDir = rootDir.getCanonicalPath()
 }
 def modulePrefix = ':'
 if (gradle.ext.has('exoplayerModulePrefix')) {