Use same debug signing certificate for all builds (#4423)

Task/Issue URL:
https://app.asana.com/0/488551667048375/1206850007250848/f

### Description
Adds a keystore to the repo and configures the gradle to use it to sign
`debug` builds. The reason for this is so that we always use the same
certificate for all debug builds, and therefore the SHA of the signing
cert can be added to the allow list for `CredentialManager`.

### Steps to test this PR
- [x] Make a debug build (e.g., `./gradlew assembleID`)
- [x] Run `apksigner verify --print-certs PATH_TO.APK`
    - [x] Verify `CN` is `Debug Builds`
- [x] Verify SHA-256 hash is
`17d41f7a9725a87572d86a427fa13b4ad411eaa4c8a78ee31d64eff62609323f`

app/build.gradle

@@ -72,8 +72,15 @@ android {
     ksp {
         arg("room.generateKotlin", "true")
     }
+
     signingConfigs {
         release
+        debug {
+            keyAlias 'android'
+            keyPassword 'pwpwpw'
+            storeFile file('debug-dummy.keystore')
+            storePassword 'pwpwpw'
+        }
     }
     buildTypes {
         debug {