gradle: make debug apk builds debuggable

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2320>

gradle.properties

···
 android.nonFinalResIds=false
 android.nonTransitiveRClass=false
 android.useAndroidX = true
-
+org.gradle.jvmargs=-Xmx2048m

src/xrt/targets/openxr_android/build.gradle

···
             // Gradle plugin produces proguard-android-optimize.txt from @Keep annotations
             proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
         }
+        debug {
+            minifyEnabled false
+            debuggable true
+            jniDebuggable true
+        }
     }
 
     externalNativeBuild {
···
     }
     buildFeatures {
         buildConfig true
+    }
+}
+
+androidComponents {
+    onVariants(selector().withBuildType("debug")) {
+        packaging.jniLibs.keepDebugSymbols.add("**")
     }
 }