CI: development builds are now signed with publically-shared key

This commit adds keystore which is shared with community. People freely can
use it for creating own Termux app and plugin builds. However, builds signed
with such keystore cannot be proven as official.

Signed-off-by: Leonid Plyushch <leonid.plyushch@gmail.com>

app/build.gradle

@@ -18,12 +18,25 @@ android {
         versionName "0.75"
     }
 
+    signingConfigs {
+        debug {
+            storeFile file('dev_keystore.jks')
+            keyAlias 'alias'
+            storePassword 'xrj45yWGLbsO7W0v'
+            keyPassword 'xrj45yWGLbsO7W0v'
+        }
+    }
+
     buildTypes {
         release {
             minifyEnabled true
             shrinkResources true
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
         }
+
+        debug {
+            signingConfig signingConfigs.debug
+        }
     }
 
     compileOptions {