Use absolute file paths for APK & keystore in Drone CI (#429)

## What changes were proposed in this pull request?

Use absolute file paths for APK & keystore in Drone CI.

## How was this patch tested?

CI/CD.

Reviewed-on: #429
Co-authored-by: Amab <juanmi1982@gmail.com>
Co-committed-by: Amab <juanmi1982@gmail.com>

.drone.yml

@@ -117,7 +117,7 @@ steps:
       # Encrypt keystore & set $RELEASE_KEYSTORE_BASE64 environment variable in Drone CI
       # - openssl base64 -A -in keystore.jks
       # Decrypt keystore
-      - echo $RELEASE_KEYSTORE_BASE64 | base64 -d > keystore.jks
+      - echo $RELEASE_KEYSTORE_BASE64 | base64 -d > /drone/src/SWADroid/keystore.jks
       # Assemble APK
       - ./gradlew assembleRelease
 
@@ -127,7 +127,7 @@ steps:
       api_key:
         from_secret: gitea_api_key
       base_url: https://git.cuernodehipnos.es
-      files: SWADroid/SWADroid/build/outputs/apk/release/SWADroid-release.apk
+      files: /drone/src/SWADroid/SWADroid/build/outputs/apk/release/SWADroid-release.apk
 
   - name: rebuild-cache
     image: drillster/drone-volume-cache

SWADroid/build.gradle

@@ -82,6 +82,8 @@ android {
                 signingConfig signingConfigs.release
             }
         }
+    } else {
+        project.logger.warn("RELEASE_STORE_FILE is not defined. Skipping signing step")
     }
 }